First, check your contract. If it's Germany, it's probably written down.
Assuming there is no agreement whatsoever (and I doubt that) then you would be free to redo the work you already did with the knowledge you now have. Redo means you have to do it again with the tools you have available now. No copying of source code files just because they forgot to revoke your access, not using their tools.
A simple example: if you program an e-commerce platform in C#, then switch jobs, you are free to program another e-commerce platform. In C#. Just imagine if you weren't allowed to do that. I programmed a linked list for my very first employer, can I really be banned from writing such data structures in the future? Obviously not.
Exceptions are (and I cannot stress this enough) your contract and any laws that apply, like patent laws. If that e-commerce site was such a miracle that they had it patented, then you are not allowed to copy it, whether you previously worked there or not.
If your boss though it would be great to sell shirts next to socks in that web shop? There is nothing prohibiting you from opening your own business and doing exactly that. Except for maybe your contract.