Check out example codes for "unity multiplayer". It will help you in understanding the concepts better.

Code Example 1

// Best free solution:

// Mirror is open source.
// Mirror is a high level Networking API for Unity, built on top of the low level Telepathy library.
// Mirror is built and tested for MMO Scale Networking by the developers of uMMORPG, uSurvival and Cubica.
// Mirror is optimized for ease of use and probability of success. Projects that use Mirror are small, concise and maintainable. uMMORPG was possible with <6000 lines of code. We needed a networking library that allows us to launch our games, period.
// With Mirror, the Server & Client are ONE project (hence the name). Instead of having one code base for the server and one for the client, we simply use the same code for both of them.

// Unity Forum : https://forum.unity.com/threads/mirror-networking-for-unity-unet-replacement.425437/
// Github  : https://github.com/vis2k/Mirror
// Discord : https://discord.gg/N9QVxbM

// Best paid solution:

// Perfect for FPS/TPS, Action Games or Battle Royale
// https://www.photonengine.com/bolt

Code Example 2

Since unity multiplay is now obsolete, here are some helpful resources:
https://www.photonengine.com/pun - The multiplayer engine
https://www.youtube.com/channel/UCyoayn_uVt2I55ZCUuBVRcQ - A channel that can teach you how to make a basic game

Everything u need can be found on YouTube

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