Remember that in order to play the game together, all players must pick the same type of game (either the full game or the demo).
The File Explorer lets you edit the game config files
(autoexec.cfg, q3config.cfg),
install game mods, and do other fun stuff.
For example, to be a true pro, change the field of view
by adding a line set cg_fov 110
at the end of autoexec.cfg.
https://web.archive.org/web/20250421121019/https://forum.fpsclassico.com/quake-3-config.php is a nice guide on Quake config variables.
For a list of mods, see https://www.moddb.com/games/quake-iii-arena/mods.
Back up important data, such as replays (demos) and configs! Otherwise it might get lost after some weeks of inactivity, depending on which host app (messenger) you're using. See this forum post about this.