Travel without accommodations booked?

I know a lot of people travel without booking accommodations in advance, and I’m curious how that actually works in practice.

What’s your approach? do you book the day before, the day of, or just show up and ask if there’s availability? Have you ever run into issues with places being fully booked?

I’m usually a big planner, but I’m trying to be more flexible for an upcoming trip to Vietnam, so I’d love to hear how others handle it.