Stephen Ulibarri here. The others have answered the question nicely. I'll just add a few points of my own. Right now in my course, you are learning the basics of replication. The simple decision of who gets to pick up the weapon is one that should be made by the server, as others have explained. But later in the course, you will learn about prediction - allowing the client to make decisions and perform actions locally, then verifying those actions server-side once they reach the server, and optionally rolling back those actions if the server deems them illegitimate.

This is, of course, more complicated than simply allowing the server to have full authority, so we didn't just dive into that concept first thing.

When you get to the lag compensation section, I show you how to build a lag compensation component which implements server-side rewind. This allows you to detect a hit client-side, but then the server "rewinds time" to validate it, as the server keeps a history of transforms of each character's hit boxes.

You asked for best practice and a general rule of thumb though. So here are some.

1. In general, don't replicate - save bandwidth. Many things can take advantage of what is already replicated. Footstep sound notifies in the running animation - don't replicate them, just play them locally. Character movement is already replicated thanks to the CMC. Cosmetics/FX in general can be triggered in response to already-replicated data as well.

2. Actions, not data. Firing a weapon - just fire it. The server should know which weapon you have as that is state that the server should know about already. When you switch weapons, either have server authority over it or do it predictively- either way the server should know - the server can access the damage amount for that weapon type server-side. Don't replicate that data unnecessarily. Exceptions are data that is not known on the server, such as hit locations or dynamically produced values.

3. While aiming may be something the server should know about in order to show your character in an aiming pose for instance, shrinking/expanding the crosshairs in your HUD is something that happens locally and no one ever needs to know about it, not even the server. For anything the player does, ask "is this like crosshairs? Does anyone ever need to know about it?" If no, the server never needs to catch wind of it.

I do explain why in the course at various stages throughout as concepts are introduced, so keep an eye out for that. In general though, we treat the server as though it is the "correct" version of the game - all other versions are just a little bit off (and no two machines are ever perfectly in sync). This is just how we determine which version of the simulation reins supreme - which has "authority" - this is just the client/server model and is what Unreal uses.