Does your company document & maintain API? If yes, How?

This question is more focused toward startups.

A single developer single handedly built entire backend over the two years and poorly documented everything during the process. Now he has left the company and there's so much chaos while working with APIs.

My work mainly involves UI integration and hence I struggle a lot to do basic work. Sometimes I end up spending hours debugging and end up realizing this parameter expects a differ type of value than I was sending.

Recently I got frustrated so much and decided to clean up everything and write well structured documentation.

I want to know how other companies manage this. - What tools and services do you use?

• When do you do it? How do you do it? Do you maintain it often?
• Is there any guide/reference/open source project that I can refer to get better understanding of the whole process?

Any additional help appreciated.