How does it work, that Tor Project alone says which nodes/ Relays are malicious?

There's different types of malicious nodes - Some for tracking, some for redirecting people to fake HTTPS sites (there was a case where an exit node was scamming Monero users by sending them to a fake exchange page, etc. Avoid this by using onion URLs where possible). There are undoubtedly some malicious tracking nodes, but as long as some of the network is legitimate, Tor can be secure.

How many bad/malicious Nodes are there in Tor? Are there good guesses?

Hard to say, but to be honest it's not hard to spin up new nodes on a docker image through DigitalOcean so a temporary attack on Tor that controls a large part of the network would be pretty cheap. That said, there haven't been any big court cases where someone was deanoned through Tor malicious nodes, so if it's good enough for criminals, it's good enough for regular people too

Why does almost noone use my Snowflake Proxy which i got in my browser as an extension?

What are your stats?

Is Tor going to implement fake traffic, so global surveilleince can’t break Tor easily because of the similarities of the traffic?

That's generally not helpful, real traffic is better than fake traffic