How do you onboard new team members to a large legacy test suite?

We have a 1,200-test suite built over 4 years. Good test coverage, but: - No documentation of test strategy or architectural decisions - Inconsistent patterns across different services (each SDET had their own style) - Several fixture layers that are confusing to navigate A new SDET is joining in 2 weeks. Last time someone joined it took them 6 weeks before they could confidently write tests. What documentation or tooling has helped you onboard people to complex test suites faster? I'm thinking a CONTRIBUTING.md for the test suite, but I'm not sure what to put in it beyond the obvious.