There is a Modem/Router from another company. If I have access to the SSID and Password, but don’t have direct access to any configuration, can I set it up so a Wireless Mikrotik can get internet from it and give internet to a camera? If so, what would I have to do? Would this be a bridge/station setup? Would I need two mikrotiks, one connected wired to the modem from a different company, and one close to the camera, or would one mikrotik be enough?
One Mikrotik would be enough.
You want to use station pseudobridge mode (or possibly station-pseudobridge-clone).
If you have two Mikrotiks, you can use station bridge mode, but if there is a single device connected to the station, there should be no difference in practice between the three and four address modes:
https://help.mikrotik.com/docs/spaces/ROS/pages/122388518/Wireless+Station+Modes
The mikrotik would have dstNAT address translation so I can access the camera with VPN and a port. Would this still be possible?
If you use the Mikrotik as a switch you need to use one of the available (station-) bridge modes, if you are going to use it as a router, then you want to the “plain” station mode.