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Please tell me what I need to do to make an online multiplayer game which can b played locally as well as globally. I have some experience in c# and unity. Please guide me
There are multiple ways to handle networking. There are many questions you will have to ask yourself like: p2p vs dedicated server, what platform does it need to run on, what kind of network transport layer am I going to use, etc...
But to start with, what I personally like is to use Mirror. It's a high level networking API for Unity and it's developed using the KISS principle (keep it simple, stupid). There is a large community using it, and there are quite some examples and tutorials out there.
You can't really just jump right into making a Multiplayer game without fully understanding what you're doing. Before trying to dive in, you need to try and understand the basic concepts of what you're trying to do with your game: What are your requirements? Dedicated servers, Peer-to-peer, Host-client? Once you narrow down your basic structure you need to understand some basic networking principles, understand how data is transmitted and what data you need to send/receive. You can use a High Level API like Mirror which serves as a surface-level introduction with super simple implementations, but if you still don't understand the fundamental concepts you'll get lost really quick and end up browsing Reddit/Forums for solutions all the time. TLDR: Learn the fundamentals of multiplayer games before trying to make them.
I would look into the Photon Pun SDK. Here is the photon doc https://doc.photonengine.com/en-us/pun/current/getting-started/pun-intro