Regarding other BASIC versions: it's probably-abandonware rather than open source, but the (more than slightly inscrutable) source code for Acorn's 8-bit BBC BASIC is here: https://github.com/stardot/AcornDmosBasic (there are commented disassemblies available - you would probably find those more illuminating...)

(BBC BASIC is pretty horrid by modern standards, but you'll probably still like it more than Microsoft BASIC. Has arbitary length variable names, multi-dimensional arrays, integer variables, PROCs, FNs, and an inline assembler.)