Developer Guide
Lite XL has little functionalities without plugins — anything that can be implemented as plugins can and will be implemented as one.
Lite XL is mostly written in Lua 5.4. If you want to write plugins, you need to know how to write Lua. There are various resources such as the Lua 5.4 Reference Manual, Programming in Lua, Learn Lua in 15 Minutes, Lua Tutorial at TutorialsPoint and Lua Tutorial at lua-users.org.
You will also need general programming knowledge and some experience with object-oriented programming.
Some of these tutorials target older versions of Lua and may be less relevant.
The latest version of Lite XL uses Lua 5.4.6, while older versions uses Lua 5.2 and LuaJIT. These versions have breaking changes that has to be accounted when following the tutorials.
Overview
Lite XL is a fork of lite. As such, a lot of concepts used in lite is inherited in Lite XL. Lite: An Implementation Overview is an excellent article that explains many concepts about lite.