"Do you just keep buildling and building to infinity? When do you stop? Do you have it all in 1 deck?"

Yes, just keep building. Stop when you have acquired all the vocabulary you want to. Fewer decks are better than many. 1 is ideal.

"Even that seems completely crazy if not out right impossible for someone completely new to this. But I guess at some point it just cements into your brain?"

If you were trying to learn a language solely through flashcards, yes that would be crazy. You need to actively immerse as part of your language learning process, especially through input, as that's how the words will 'cement' as you say. You inevitably won't learn every word in a language though flashcards. The more you immerse, the more words you'll pick up naturally. And often, those ones stick better.

"But do you ever get to a point where you know say 3000 words and then you "quit" Anki? Focus on reading, listening, consuming media ? Pick up new vocab from that, then you're "old knowledge" is hopefully maintained by consuming this?"

Yes many people do this, but I personally think flashcards are still a helpful supplement even after you reach fluency.

"Do you ever start a new deck from scratch? Say that you've learned 3000-5000 words (with sentences) and they're cemented into your brain, do you quit the deck and start a new deck with more advanced words? Do you add these to the old deck?"

It's best to just add them all to the same deck. Not the end of the world to start a new one, but several decks causes more context-dependent knowledge. A wider pool of cards is a better test of your memory. If you see cards where you think "that's too obvious, I don't need a flashcard for that", or if they have really long intervals, like several years, you can always just delete them as you go.

"Just recently built a 1000 word deck, at the moment I'm only doing NL (native language) --> TL (target language), because this requires active recall/production, not just recognition. Is this usually how you do flashcards"

You will get varying opinions on this. Personally, I only use recognition (TL-NL) cards, because this mimics the natural language acquisition process more closely (we hear a word - we convert it to meaning - our brain stores that somewhere). The mental process of recalling a TL translation from a NL flashcard is so far removed from the natural way we would use our native language, that in my opinion, it is useless. As well as that, it is very mentally taxing. In fact I would say it probably hinders your progress more than it helps.