It's amazing how it happens. I think it comes down to someone has to keep an eye on the kid at all times until it gets to a certain age and you generally trust the kid. It's unfair for one parent to keep an eye all the time so there's a lot of unscheduled switching off.

At first, kids need basically 100% attention & surveillance, except when they’re asleep.

So between two people, one is occupied with a kids almost all the time.

When they are born it’s really chaotic. All the stereotypes apply: sleepless nights, etc. after they grow, it gets easier. After my kid was past that phase it was really smooth. My wife and I got our hobbies, had time to do stuff we liked, etc. It all comes down do partnership. We had our second after our first was in his early teens and all the chaos is back and it’s a bit harder now because its 2 but we still manage to have our private time, although it’s been limited.

tl;dr: it shouldn’t be time consuming.