Base AP is TACC and single-lane autosteer.
You can turn either on anywhere.... though the manual explicitly states autosteer is only intended for controlled access divided highways.
FSD is a package of features (which required you to already have base AP before you can get it) that works in a bunch of different places (depending on the specific feature you're talking about)
On the highway it adds lane changing and navigate on autopilot for example
In the city it adds (in really early beta as of only a week or two ago) stopping for lights and stop signs.
Anyplace it's relevant it adds auto-park and summon.
(eventually FSD is promised to also add what they're now calling "autosteer for city streets".... technically you can turn AS on right now on city streets but it's not intended for that use and has no idea how to handle intersections, turn lanes, roundabouts, or anything like that)