It usually is the installer but sometimes is just the launcher. Ive bought Ubisoft games on Steam and it will install via Steam, however when I launch it it will open up uPlay (downloads it first if its not already installed on your PC) then launches.
Basically no matter what the launcher is the most important thing it does is act as a copy right protection DRM.
Yes all people will see one another no matter what the platform is as long as they are in the game, except for console players although some games do have cross-play. The biggest issue is friends lists, with each separate launcher they have their own social environment, some allow you to sync with Steam some require you to make a separate account (username, password etc).
Personally I always have Steam open no matter what game I'm playing, its the biggest platform for PC Players and has text and voice chat functionality built right in then I link up in whatever game/platofrm to play with friends. Some people also tend to use Discord for their friends and chat then link up in whatever game they want.