Can women use bikinis as underwear?

Cause isn't it almost the same thing? I know underwear can be more revealing but isn't it basically an underwear, because it comes with a bra too or is that not a bra? I'm thinking of the difference between female underwear and bra and a 2 piece bikini and it looks basically like it's the same thing with the same function, is one more comfortable than the other or what's the difference? Lol
Can women use bikinis as underwear?
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