Effect of whirlpools on physical recovery?

I’ve seen a lot of claims about the general health benefits of using a hot tub/whirlpool or sauna, particularly after a workout, but I’ve yet to see an explanation.

Are there clear benefits If so, what?

What causes them? Increased surface blood flow?

Is one (Sauna/Whirlpool) more effective?

Is there an ideal time limit to shoot for when spending time in one?

I’ve been using a whirlpool occasionally after my workouts because it’s a pleasant, relaxing sensation - but it tends to make me very weak for a short while. The walk from the tub to the showers seems to require a huge amount of energy … but after a while, I end up feeling more refreshed. What’s up with that?

I was a lifeguard for a while and part of that job was to inform people that they’d been in the hot tub for too long. The recommended time for any of those (hot tub, sauna, steam room) is ten to fifteen minutes at the MOST. So many times I encountered people standing up out of the hot tub, getting light headed and almost passing out. Warmth loosens muscles so it can help alleviate strains and pulls, knots in the neck and shoulders. However, if you have aching bone pain or a headache, warm water can actually make it worse. Stiff backs due to arthritis or disc pain should be treated with ice, as well as migraines and tension headaches.

Were the negative effects of staying in too long relating to blood flow issues? Too much on the surface, not enough in the brain (leading to passing out, etc)?