FOSS gamedev and creative worlds

so id like to make a new portal shape. for use w/ pilz adams nether mod. how do i make schematics is there a software or what

See WorldEdit mod.

In particular, its `//save` and `//mtschemcreate` commands (docs, tutorial).

thx i'll check that out

Best way is by Lua tables, this gives you full control over things you cannot set using worldedit, such as per-node force-place.
Use the method of this mod https://github.com/minetest-mods/saveschems
Enable mod and when entering the world the mod will run and save schematics to files.