ACA income minimum limits-what if not enough income in 2024

For 2024, I'm helping my adult grandsons (college students ages 22 and 23....currently not working but hope to) get their own medical coverage after their coverage with their Mom's COBRA plan lapsed last July(her error, but too late now).  I'm  looking at coverage for them through the Marketplace but if we chose that, and they take the advance ACA credit:

...is there a minimum income requirement for ACA credit for 2024?  Is it calculated annually or monthly?

...what happens if they don't meet the minimum income requirement?  Do they lose the credit entirely and have to pay ALL of the advance back or just part of it?(like what if one only made $10,000 for the year and the other one didn't work at all)

...neither are employed at the moment, but hope to get jobs(one is part time student, other is full time student)  One has a paid summer internship....assuming it actually happens

***I think I need a crystal ball here 🙂 and answers on the internet are hard to find.  To be clear, I prepare the taxes for all of my various family members, am pretty familiar with the tax code(at least the parts I have had to use) but this isn't anything I've ever needed to know.

***please help me.....I'd hate to advise them wrong and have them be faced with a big tax bill at the end of the year!