Bags checked to final destination question

While I wait on hold for a million hours to ask United this question, I thought I’d ask here.

I booked my MIL on a flight from COS to DEN, then separately from DEN to STL.

Can she get her checked bags sent all the way to STL from COS or does she have to retrieve them and recheck them in DEN? Can she just ask the baggage check in at COS to do that?

3 of us are all going to STL together from different starting points and I wanted to book the DEN to STL all on one booking.

Additionally, I booked it with my husbands United credit card. She’s in coach COS to DEN and Business/First DEN to STL. Will she have to pay for baggage COS to DEN?

We are fairly newbies to United the last couple years so I’m still pretty unsure on all their policies.

Thanks in advance.