Or, I've seen (and done) a charity donation link myself for my birthday. Still asking for cash but goes directly to the charity so a bit less cheeky.

I don't blame you for being mortified tbh. I've never heard of it and i'm only slightly older than them. Do you think you might be a bit soft on them by any chance? Referring to your older two as young adults is a bit of a stretch?

Very poor taste. And why are 3 adults still looking for money from mommy?

Never really heard of this. But I'm sure there's a lot they do they never tell you that might make you feel uncomfortable. I'm not clear on what these "money apps" are. Is it like Paypal or Venmo? What is the context in how they are "begging" for money? Are they content creators doing livestreams or something where fans can send money if they choose? Is this them telling people they would rather take money where they can decide what to get themselves, instead of being bought gifts they may not want?
At this point in your adult daughter's lives, they have free-will to make their own choices in life, and do as they choose. If what they are doing is socially unacceptable to general society, people will let them know. If what they are doing is socially acceptable, then it's a good idea to back off a bit. As long as they are not scamming people with lies and fraud.