Generically speaking you have...
backing on your reel.
then anywhere from 50ft to 50 yards of running/shooting line.
then your skagit or scandi shooting Head.
Then your tip or poly leader. Floating, slow sink, or fast sink.
Then 3-4 ft of tippet.
Then fly.
Or you can get an integrated line that combines the running line and shooting head just like a regular fly line. Advantage of this is that you don’t have knots going through your guides when stripping in close...which is never pleasant. The disadvantages of the integrated line is then you can’t switch heads and it probably won’t shoot as far.
It seems complicated but once you get the terms and lingo figured out it gets a lot easier. It certainly feels like drinking out of a firehose in the beginning but give it time and you will figure it out. Lots of great videos out there...Ashland fly shop, reds fly shop, Tom Larimer, Simon Gawesworth, as well as a bunch of others. Highly recommend watching them...helped me a ton.