We tend to thoroughly document our API (which is the backend behind our mobile apps and website) using ApiDocJs.com or Swagger.io&#x2F;swagger-ui Every service and method is thoroughly explained in detail so the front-end folks have a reference to work off of.<p>The rest of the systems are documented ad-hoc. Some readme files here and there, a large block of comments inside of confusing files, the occasional style guide, etc.<p>We also have an onboarding guide for new devs (just a PDF) which walks them through our systems, our tools, etc. Nothing fancy, about 10 pages.