Motorcycles have mounts (on the swing arm orunder the tail light), if it is a pedal bike, most are just zip tied to the bottom of the seat.
yes but it do not need to be there
No, you do not need a license plate for a bicycle.
take off the license plate then screw the holder on with the plate.
on the bumper?
Yes
Yes you can!
yes
If you're asking about where to put your sticker on the license plate than that would be the rear plate.
Probably
You put a sign in the license bracket "in transit"
If it comes with a plate mount it will be under the rear of the seat or by the back mudguard (if equipped).
the top right corner