Changing the transmission fluid is not always a flush, although a flush does change the fluid. A "flush" is often what they call it when they power flush a system with pressure, often something not recommended unless done from new, and regularly. It will change all of the fluid.

I have changed my fluid in several vehicles by pulling the transmission lines going to the radiator, and doing a fluid exchange. It pumps the old out and puts the new in. You can get basically all of it that way. In older vehicles like a 98 F-150 I had, you could drain the pan, and the torque converter easily, changing nearly all the fluid.

Many do a drain and fill approach, draining the pan that gets about a third out. It is the easiest to do, and in some situations the safest.

In my 2013 prius, it had a CVT, and changing the fluid was super easy, like doing a differential. Just drain it, and fill it, one bolt for each. It drained it all out. Pretty much as easy as a drain and fill.

So many different ways you could use to "change"  the fluid.

What transmission does yours have, and how many miles?

Change the FAQ for more transmission info.

Both your questions are covered in our FAQ