A long time ago, Disney had a booklet that told how to enter different attractions/mobility entrances. At that point, many attractions were not accessible thru the regular entrance, so there were special instructions about which door to use, some where you entered thru the exit, etc.
A lot has changed since then. Most attractions have what are called Mainstream Lines, which means guests with disabilities enter in the same place and use the same lines with everyone else. As attractions were built, they were built with Mainstream Access and it was added during renovations as much as possible.
There are some attractions where guests with disabilities will use a different boarding area, but in most of those, you will still enter thru the regular entrance and wait in the same line as other guests. Closer to the boarding area, there will be a ‘pull off’ that leads to the accessible boarding area. Those are either well marked or a CM will be ready to direct you.
Animal Kingdom and Disney Hollywood Studio were built with Mainstream Lines. Epcot is Mainstream except for Spaceship Earth.
Because MK is older, there are still a few attractions that could not be changed to Mainstream. For those few, you will mostly be given a return time to come back if you do not have a Fastpass for the attraction. Off the top of my head, those are Big Thunder Mountain, Small World, Jungle Cruise and sometimes Space Mountain.