Carmack talked about how they released those first games on the Lex Friedman podcast. Having a previous framework and pipeline seemed to help get them out really quick.
He talked how they were always working on 2 games and would do a press release with a shipping date before they more or less even started lol
Romero just released a new book 10 days ago... Doom Guy: Life in First Person https://amzn.to/43M0rSg
The crazy part was also working 2 full time jobs essentially, 8 hours at softdisk then they would use those PCs at night for 8 hours to crank out their private games, also borrowing them on the weekends to get more work done.