It all depends...
luatex is a two-language system and a lualatex package can use either language.
You could write a package written just as a classic latex package but using some of the extended tex commands available in luatex, such as
You could write code that is almost all Lua, with just a very thin TeX wrapper
mypackage.sty that just does
luaotfload comes to mind)
There isn't really a lot in common between these kinds of packages other than that the end user doesn't necessarily need to know anything about the implementation language, and can just do
In one case, The coding practices are just as in classic TeX, in the other case, the coding practices are those of Lua (mostly).