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Like this: https://dribbble.com/shots/4414790-Modal-login-screen-Sketch-freebie
I never see them on top.
Is there a reason?
It is about how to read content. While putting the form above, you emphasize your own login system over the others.
In mobile, buttons such as log in are placed where users would expect it to be. It can also be stated that, buttons being at the bottom is easier for users reach when the phone is held in single hand
Fitts law applies here
Required, but never shown