At Salesforce, we have a mix of different techniques. Along with standard best practices such as documentation, mentoring and code reviews we also have more "corporate" resources.
We have an onboarding program that requires all world wide employees to attend some classes at our HQ. Developers follow a dedicated 2 weeks program. This allows them to become familiar and align themselves with our tooling and Agile methodology (Scrum, Kanban...).
We also have a very valuable tool to onboard developers: Trailhead our e-learning platform.
We use this tool both externally to grow our ecosystem and internally to onboard our staff with some private content.