Not using things like go to definition and any fancy tools, just manually forcing myself to work through files to understand how things fit together and using basic tools like grep
Very surprising! IIUC, you consider "Go To Definition"/"Go To References" and other "LSP assists" unhelpful (or worse) when familiarizing yourself with a new codebase. I personally find them indispensable. Could you say more to help me understand your position?
If I start with these tools I get a feeling I understand how things fit together, but when I want to add something new I realize my understanding is pretty shallow. I force myself to manually open files where things are so I get an understanding of the conventions and principles in the project. By far my most valuable tool is grep (it’s super fast in vim) and I grep the code to see where certain functions and such are used. I use the lsp tools later when I’m used to the project.
I use "go to definition"/"find references" to go all around the code base and at the same time try to figure out how each files interact with each others.
I would prefer LSP assists as mentioned above. I love having things within reach. I believe programming should be as painless as possible.
Pharo(Smalltalk) IDE is one of the best IDE I've ever used. It even has a feature where you give input object and output object and it tells you what messages you need to send to get from input to output.