So,
a little update on this.
After a few calls with Apple Support and trying to reset NVRAM and SMC countless times, they told me to send the laptop for service.
So, Wednesday night I went to an authorised Apple service shop and explained the guy what happened.
What he wrote on the report was:
Computer is not starting after Big Sur update. Apple logo lights up. Black screen.
I told him I had a case with Apple Support, but he told me he didn't need that.
Thursday morning I got an e-mail from then saying this:
Software upgrade is generally not cause of a pre existing Hardware issue.
Vin device, apple will not perform repairs, repair payable
Today I went there to get my laptop back and talked to the guy. He told me my computer is considered vintage and Apple only holds parts of up to 6 years old laptops and therefore, they cannot fix my laptop for me. If I pay them to fix it, then fine, they can do it.
Conclusion is... Apple broke our laptops and they won't do anything about it. When they released Catalina, some T2 chips were corrupted and they also didn't fix it.
I read some news about an Apple engineer saying the issue is with the HDMI IC and once he removed it, the problem disappeared.
Some people are saying you could reflash the "BIOS".
None of those solutions are something you can just do at home without special equipment.
As usual, I hope you get a copy of this via e-mail, as I believe this comment will be deleted soon.