I often do work without direct internet access and it's painful at times. PDF documentation is wonderful, but when that's not available I sometimes save entire websites. Other useful things: dictionary, thesaurus, encyclopedia, world maps, reference books. It was surprisingly difficult to find a free dictionary file.
Overall it's not very different, just inconvenient at times, and the lack of distractions is nice. I echo your wish for local-first programming.
At the beginning of lockdown last year I thought it would be a fun projects to build a doomsday-type local internet. Just a raspberry pi with it's own AP hosting various offline version of reference websites.
I thought it would be pretty low effort: maybe a few scripts to crawl some websites and then a few scripts to spit out html. I never got passed acquiring all the references. Most of the available stuff is from over 10 years ago. Wikipedia's convenient snapshot are from 2008, the dictionary files I found seemed to be based on Merriam Webster from 2009 and I couldn't find a good reference for scientific unit conversions just to name a few.
In the end I shelved it since it was going to be much more than quick project. But I must say that i'm impressed with Gutenberg project on how they provide an option to download all their books.
Overall it's not very different, just inconvenient at times, and the lack of distractions is nice. I echo your wish for local-first programming.