How do I start and login to my iMac when Bluetooth is off

Use a wired mouse and keyboard instead. If you have a magic keyboard without batteries you can also connect this via the charging cable. this will become difficult with the Magic Mouse tho, you cannot use that one when it is charging, so I recommend a "normal" wired keyboard and mouse. Hope you have one of these lying around!

Use the cable that was included to connect your wireless keyboard, then it will allow you to login.

Do the same (if it’s possible) with your mouse (will be tricky if it’s the Magic Mouse).

You’re welcome! 😁