BitLocker automatically enabled after new Win 11 install?

[mfessler 0]

I just upgraded my brother's PC with a new M.2 terabyte drive and Windows 11 pro, but left his Windows 10 drive in place so he could transfer files. I did have to change the BIOS settings to get Win 11 to install. He told me he needed to boot back into Win 10 to run a program he doesn't have on Win 11 and when I changed the boot choice, and let the computer boot up I got a blue screen telling me BitLocker would require a key. Since we made sure that the check box options were turned off and did not create an MS account during this fresh install on a new drive, this was a big surprise. I really don't want BitLocker running, but it has already encrypted both hard drives. I found some threads about disabling BitLocker, which I will follow when I am back at my brother's house. What I would like to know is how would he be able to get a Key for Bitlocker, if we never asked to have it enabled or logged into a MS account? If it ever gets enabled again and we need to do something that requires the key, I want to have it stored somewhere. Lastly, if I just go into the settings and disable BitLocker, will it automatically un-encrypt both drives?

For now, he's just using Win 11 until I can get this figured out.