Videos you host will never get ads, unless they’re baked straight into the video file, (and at that point you need to reconsider where you downloaded them from.) Plex does have a sort of IPTV service that they run, which has ads. But most Plex users tend to ignore that part of the app, because that’s clearly not why they are using the server. Just use it for hosting your own stuff and you can ignore the Live TV/Movies & Shows stuff from Plex entirely.