my iPhone 12 bluetooth is not detecting bluetooth devices

first time you connect a bluetooth headset to a device you are required to put the headset into pairing / discovery mode

how you do that should be in the manual or on the website of the manufacturer of the headset

If they are auto connecting to those devices they will not be visible to your iPhone. Temporarily turn off BT on both, put your headset in pairing mode, and try again.

As stated put the headset into pairing mode https://www.techlicious.com/tip/pair-onn-headphones-to-bluetooth/