I would wager most of them had a core that worked the same as described in the Usborne 1980s computing books. Even ones not written in BASIC (like the Usborne gamees) were doing largely the same thing in asssembly language (faster) or in their own slightly higher-level scripted language.
You have a database of locations and their connections, a database of objects with flags/variables and descriptions, and a list of verbs. You match verbs to objects and program consequences for each pairing that you care about, and built a standard model here and there for things like TAKE/DROP.
Here’s a link to the freely downloadable books:
Scroll down, especially, to WRITE YOUR OWN ADVENTURE PROGRAMS at the bottom. There are also two book-length adventures there, Island of Secrets and Mystery of Silver Mountain, which also have a lot of description of how they work.
I used these books to write a bunch of my own adventures. If you want to see what kind of planning was involved (lots of writing lists of things on paper…) have a look at my blog where I’ve got up the scanned notes for several adventures:
(sorry, no https there, but it’s safe.)
(years with the most adventure games and their notes are 1988 and 1990)