Question

Is there a guide or explanation for how classes written in as files are compiled by assemblyscript and usable from there ? Are constructors, static methods, ordinary methods ok and under which conditions ?

The as-bigint package and repo uses them, and apparently it compiles under their obsolete version of assemblyscript and not under current version – and I could not get an explanation for how that's possible.
The as-mpz seems to do the same, and while it uses a more recent version of assemblyscript to do so, the code (especially the built code) is very intricate to understand for a newcomer. In particular, it is very long within a single file and so what makes it work or doesn't work is not easy to understand.
Which is also the reason for guides and documentation to exist: because if retro-engineering is not an efficient way to learn a language.