When I started making mods for the server, I was first pointed towards the dev wiki: https://dev.minetest.net
It is fairly outdated and even according to some devs, should not be used as a primary source of information. Still, I found it quite useful in the beginning and it has some example code, but seems to be less maintained than most other sources.
The modding book of rubenwardy helped me get started for real: https://rubenwardy.com/minetest_modding_book/en/index.html
It has working code examples that solve problems next to every mod developer will have now and then. The same guy made the Node Box Editor (https://rubenwardy.com/NodeBoxEditor/) and has a more or less minetest-y website: https://rubenwardy.com/. He runs the content db (https://content.minetest.net/) and the famous CTF server vps.rubenwardy.com:30001
Official Modding API
This is the one go-to address no mod developer can avoid. It looks crude, but has the most current information about the minetest api, because it’s maintained by the devs themselves: https://github.com/minetest/minetest/blob/master/doc/lua_api.txt
This link points to the latest version of the document, if you want them for a specific prior version take a look at the same position in the branch your version is in: Just replace the “master” for your version. For 5.1.0 it is https://github.com/minetest/minetest/blob/5.1.0/doc/lua_api.txt
If you want to search it, use your browsers search function.
Official Modding API in pretty
You dislike the official modding API because it is not fancy enough? Someone niced it up for you: https://minetest.gitlab.io/minetest/
It has the same information as the Official Modding API. I use it when I need to understand a distinct topic and avoid misscrolling into some part that has nothing to do with it. It also has a search built in.
While the content db itself does not have any hint on how to make mods, you can go through the source code of every mod made public on the content db: https://content.minetest.net/
That’s a great source for finding code examples. I downloaded every mod that is in there and whenever I want to look at a code example, I can be 99% sure it’s somewhere in the downloaded mods and I can take inspiration from them or weed out the issues my code certainly has or when I can’t wrap my head around some concept.
You are not alone. There are humans who make videos on modding Minetest, most of which may be outdated by the time you read this. You can still take a look at them, most things won’t have wildly changed. The youtuber I understood most of what he said was Nathan Salapat. While the rest of his website may also be of interest, you can find his modding videos here: https://nathansalapat.com/minetest/modding
There are plenty of other youtubers, if you find a modding tutorial that does not cease to exist halfway through or is at least educational and entertaining to some degree, drop me a line and I’ll include it in my list.
On twitch, there seems to be only one french guy who often streams about making mods: https://www.twitch.tv/sys4
Humans you can ask
When I have questions that cannot be answered by any of the above sources, I go bugging those who know way more than I do. You can find those helpful humans on the Unofficial Discord Server https://discord.gg/6W84ytH or on the forum in the mod-section: https://forum.minetest.net/viewforum.php?f=46
You found a source I did not list, want to add something or debate the usefulness of one of those sources? Feel free to drop me a line.