I’ve been in parties with some people and they have always had more dungeons than me in the dungeon finder. When discussing it, they mention a timeline. Do I have to do certain quests or what? I’m level 40 and all I have is BFA dungeons.
It can depend upon the Timeline you pick. For example, if you are using Chromie time, you need to be in the Cata timeline to see the old 1-60 dungeons
When you use Chromie Time, people generally refer to the various expansions as different timelines. Which expansion you pick for Chromie Time sets which dungeons you can see. (You go back to Chromie to change your expansion any time you want.) New players need to get a character to 50 before they can access Chromie Time for future alts. So, on your very first character, you’ll be limited to the BfA timeline for dungeons using dungeon finder.
Once Chromie Time is unlocked for your account, characters below 50 can find Chromie in Stormwind/Orgrimmar to turn it on/off/change expansions. She’s near the embassy in both cities, and the location will be marked on the main map with an hourglass icon. (I think the icon only shows up if you are eligible for Chromie Time.)