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Connecting two devices with different scenes

I'm implementing a game in Unity and I have a problem that I need advise for. I want to connect a tablet and a computer. The scene that is on the computer is supposed so send an event once an object is collected so that a special scene can be shown on the tablet. So based on what is happening on the computer there is a different scene shown on the tablet.
I only have no idea how to achieve that.

First, you need to create 2 different applications (for desktop and mobile). Second, you need to determine what protocol you want to connect desktop app and mobile app. The easiest way to implement this I think by using WebSocket/internet but not HTTP. Yes, you need to write code for the backend so it likes creating 3 different apps. But you can use firebase to simplify this process.
I recommended you use firestore from firebase. Connect booth app to firestore. When desktop app sends data to firestore, firebase will automatically notify mobile app and send that data. But you need to figure out yourself how to pair a desktop app and a mobile app otherwise, your firestore will receive data from all connected desktop app and send the same data to all connected mobile app.
This tutorial to use firestore:
https://www.youtube.com/watch?v=b5h1bVGhuRk
But before that, you need to add firebase to your project before you add firestore:
https://firebase.google.com/docs/unity/setup
You can use any wireless protocol available. You can even use Bluetooth if you want your app offline. But I don't know how to use Bluetooth...

Communication channel between a windows app and UWP app

I have a uwp project and a windows app (net5).
Both are installed in the same machine (same will happen in production), but the uwp is installed using sideloading, not on windows store.
The windows app gets some data by listening to a wss port. I have to pass those data from the windows app to the uwp app and also wait for a response back from it.
To do this internal communication I came across the AppServiceBridgeSamples in this repo https://github.com/microsoft/DesktopBridgeToUWP-Samples but it is outdated and I can't use it.
Is there any alternatives to it? What would be the easiest implementation to achieve this?
Any suggestions/comments are appreciated.

Currently, the general way to implement a communication channel between a UWP app and a windows app is by using App service. This is not outdated. If you don't want to use the App service, you have to build your own socket server so that both of your apps could communicate with it. Another way is that your windows app could save the data in a local file and load them in the UWP app.

I want to send a string from an android device to a windows machine over wifi with c#. Is this possible?

I want to make a program that transmits strings from an Android device, to the web or directly to my PC.
Do note that the android and windows device are connected to the same wifi if this makes a difference. The problem here is that I do not know how to send strings from one device to another. I am thinking of doing it in a txt file and then reading it with the program on my PC (Which I have already tested), but I don't know how to transmit it.

Yes, you can share a data from cross platform devices. There are many methods to do it. I would personally recommend using Web APIs using asp.net core, the consume that Web API in your android application, it can be in Xamarin, and for Desktop you can used HttpClient package to consume either in WPF or Winform.

Data forwarding server

I have the app for Android and another app for PC, both were written by me. Android app connects to the PC app, and the PC app sends data (in realtime) to the smartphone.
The problem is: if the PC is connected to the mobile wireless network, I can't get its address, so I can't connect my Android app to the PC.
So, is there some simple application (for Windows), that I can use as a server for data forwarding. PC and mobile App will be connected to this server (launched on the station with the static address), and the server will just forward data from one app to another?
Or maybe there are good tutorials for creating such kind of server on C#/Java?

I would use a third pc (a server) that acts as a bridge for Android and PC. I wrote a web service in .NET for a virtual server on the internet. Both Android and PC know the ip address (or the domain name) of the server.
The PC talk to server using .NET native web service call, the Android uses ksoap.
For more infomration about ksoap for android look here:
http://code.google.com/p/ksoap2-android/
EDIT:
I suppose you can have the web server in the pc but you need to configure your modem to forward specific ports to the pc. But if you have no static ip address you need to reconfigure the ip address on Android device everytime your modem is rebooted (or switched off/on). You can also try to use dynamic dns services...

Check this out:
http://whiletrue.blog.com/2013/07/11/smartphone-as-pc-controller/
I've made this with Muzzley. Basically, you already have most of the work done if you integrate with it.
(disclaimer: i work for this project)
On the PC side, you can use the lib to include in you pc app in several languages:
(java) http://www.muzzley.com/documentation/libraries/java.html
(.NET) http://www.muzzley.com/documentation/libraries/dotnet.html
(nodejs) http://www.muzzley.com/documentation/libraries/nodejs.html
On the smartphone side, you can use any of the existent widgets (virtual interfaces):
http://www.muzzley.com/documentation/widgets.html
or eventually make your own:
http://www.muzzley.com/documentation/widgets/webview.html
Example:
You can pull from github several examples here:
https://github.com/muzzley/muzzley-demos/
(check the the webview example)
I hope it helps.

can c# be used for communicating mobile and pc?

As a minor project in my degree I would like to communicate mobile and pc .I am interested to communicate through bluetooth is it possible to do so in c#?
If possible please provide me list of application or hardware i would requiere in order to do so.
thanks in advance

If (and it's a big if based on the lack of information in your question) you mean "How do I write an app on my PC, using C#, that can communicate with a Windows Mobile device that is connected to my PC via ActiveSync or WMDC" then the answer is RAPI. A free, open-source managed wrapper for it is here.

Yes its possible if its with windows mobile phone or pocket pc with bluetooth. You need a bluetooth enabled computer and mobile phone. For dev you need to download the windows mobile 6 sdk.
Then developing of the communication between desktop app and mobile phone app, you can use the 32feet.Net is open source.
When you download it and installs (well it unpacks to program files folder) you will have the dll that you make reference to in Visual Studio. Also you will get some sample apps. One of them is a Bluetooth Chat that works in Desktop, Pocket PC and SmartPhone.

Another option you could use is set up a web service that acts like an intermediary between the device and your machine. One huge benefit of this is that no longer are you bound by the distance requirements of bluetooth.
Of course if you're trying to build some sort of proximity service (do z when item x is 3 meters from item y) then bluetooth is probably the way to go.