There are two kind of mods, server side and client side mods. Server side mods are usually mods that change game mechanics. Valheim plus is serverside mod. All players need to have it to be able to connect to the server.
Client side mods are mods you can use always, server does not need to have it. For example, farm grid and mass farming are client side mods.

My suggestion is to use R2modman from thunderstore. It is separate application which you can use to install or remove mods. It will not mess up your vanilla files because all the mods will be saved in separate folders. And you will have an option to run modded or vanilla game.
Also, my suggestion would be to use only one bigger/more complex mod if you don’t have experience with mods. Installing multiple complex mods could make a mess from your game. Valheim plus is good to start with.