All the business knowledge is in Basecamp as that's where all the "work planning" is done. If you want to find out how a specific feature works, or how the decisions were made for that, go find the project in Basecamp, it'll be archived if it's shipped but it's still searchable.
For new dev setup/infrastructure stuff we use a github Wiki and the new starters point out issues in that for us - their first tasks are to update it if they find issues or missing info because it forces them to go and dig around to find the knowledge islands - they're the best people for it because it's who we want to solve the problem for.
Admittedly we've got a small company and a nice tight tech stack but there's still tribal knowledge in 2-3 people's heads and distilling that out to the team is a slow process of pair programming and explanation.
For new dev setup/infrastructure stuff we use a github Wiki and the new starters point out issues in that for us - their first tasks are to update it if they find issues or missing info because it forces them to go and dig around to find the knowledge islands - they're the best people for it because it's who we want to solve the problem for.
Admittedly we've got a small company and a nice tight tech stack but there's still tribal knowledge in 2-3 people's heads and distilling that out to the team is a slow process of pair programming and explanation.