Yeah, it was my problem as well.
For the most part it seems that one has to "learn" the API by exploring it and trying what works and what doesn't. This is not ideal. For now most of the API definitions you find are crowdsourced by the community (from txt files and ingame trial and error).
But there are some tools to help the development process and the community chat on Gitter is also very helpful, even a dev from ZOS is posting there. It's also interesting to look at the code of other add ons.
- See the sticky posts in Dev Tools for resources: https://www.esoui.com/forums/forumdisplay.php?f=173
- I recommend the API explorer (way faster than exploring it ingame or in extracted API files): https://sir.insidi.at/or/apiexplorer/
- And here is the thread about Gitter: https://www.esoui.com/forums/showthread.php?t=10
ZOS could auto generate the API from docstrings in their source code, but instead it seems like they do it manually, using documentation in Confluence file format and then copy it to the text file. At least that's what it looks like to me. Well, at least it gives the community something to do.
Edit: Gitter link is case sensitive so the link in the thread won't work, correct url is:
https://gitter.im/esoui/esoui