1. Some of the pressed metal ones have little tabs around the hole that apply pressure to keep the flange in place on the pipe.

2. Some have a set screw.

3. Some have nothing at all, but can be held in place with a couple of small dabs of silicone.

thank you.

I have the one that will require a small dab of silicone.

Don't over-do the silicone: you want to be able to remove it easily at some point. Once you've applied a little silicone to the back and cleaned up any oozing, you can hold the flange to the wall with a couple of pieces of masking tape while the silicone sets up.