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.)

Atari Basic source with comments was published [1] in 1983. Literally published, as a spiral bound book! Teenage me learned a ton of 6502 from that book, back when learning 6502 was hugely useful!

The book has intro chapters describing the entire design, bugs that snuck in, etc.

While this code dump is cute, and MS basic more widespread (at the time), the overdone readme and the 48 year delay make it a lot less interesting

[1] https://archive.org/details/ataribooks-the-atari-basic-sourc...