@fischer.poco
I noticed in your attempt to "fastboot oem unlock: that you received a message "Flashing Unlock is not allowed".
Do you know what model of phone you have? (e.g. within OnePlus NORD N10 5G there are 4 BE2026, BE2028 (T-Mobile), BE2029 and a 4th one (MetroPCS) I can't remember the number).
Basically, you need BE2026 or BE2029 to directly unlock the bootloader without OnePlus. If you have BE2028, you have to request the bootloader from OnePlus (which has been mixed, sometimes they give you the unlock token, sometimes they don't).