Kian's document is the best resource I've seen for onboarding to Parity and can be found here. Read it and explore everything it links. Recurse through the links that those resources include.
There is a backup link to the source here in case he takes it off his website again.
Also Shawn gave a great talk at Sub0 in 2024 about being an effective dev in the ecosystem. It's worth a watch.
The Polkadot Fellowship Manifesto outlines the expectations and expected mindsets etc, also worth a read. If you want to build the pdf I recommend tectonic which pulls in way fewer dependencies than latexmk and downloads what is needed at the build step, rather than downloading every possible latex module pre-emptively.
A bit more in-depth: you can see what developers at different ranks have been working on in the evidence repo which can be useful to calibrate yourself and how you plan to take on complexity.
A successful onboarding in Parity should leave you operating around the level of what the manifesto describes at rank 2.
General Ramp-Up Expectations