But in the original repository, the hardcoded seeds and the hardcoded hash values are also regularly adjusted or “forked” as you call it in that case, so what exactly is the difference? Because nothing else was done there.

Just because one thing happens in a different place doesn't make one thing better or the other worse in my opinion, does it?

I know these explorers that follow the old chain, but what do I want in a network where 93% of all coins are on 1.5% of the addresses used, I have nothing like the majority and there is nothing left for anyone to mine unless you have a mining center the size of a soccer stadium?

No, this explorer is not stuck, you can easily determine this by comparing the hash values, only the genesis block is the same. This chain and the network moves as a parallel in the mainnet and has been restarted, as far as I can see.

I don't really understand the whole thing yet, but I have compiled Bitcoin Core according to this documentation, am already connected to the network and have mined 15 blocks so far, which corresponds to 750BTC or is this now an error in the matrix?