I’m struggling to find mods that add to the block variation, and I’m looking for this sorta underground facility vibe. Does anyone know any mods that might have blocks like this, so I can build this kinda stuff? I’m building in creative 1.20.8! ^^

Create, especially it’s add-ons definitely have a lot of industrial style equipment and blocks that you can use. Immersive engineering and its add-ons are also good, any of these technology/industrial mods would provide plenty of blocks.
There are also mods that just add random items to the game and sometimes these are themed around modernity. Just casually browsing through the website of your choice looking for these things would save a lot of time and effort, but those are my two best guesses.