If you know C, retroforth's nga vm is ~300 lines and quite simple. I found it to be enlightening.

https://git.sr.ht/~crc_/retroforth/tree/master/item/doc/Nga.... - this is the "literate" version, there are a few variants of the source in the tree if you look.

Jones Forth[0] is a good literate programming implementation that explains the internals as he goes.

[0]https://github.com/nornagon/jonesforth/blob/master/jonesfort...