DOT rules state that for all check ins on the same ticket you get the baggage allowance and fees of the First Marketing Carrier (UA). However elite status benefits do not carry over to other airlines unless there’s a partnership agreement to do so.

So so for your Aeromar check-in you get UA”s non-elite allowance (zero, 1st checked bag $30). That’s unless I’ve missed a partnership between UA and Aeromar.

My common example is when I fly to India. Outbound PDX-EWR-BOM all UA I get 3 bags @32kg. But returning BLR(9W)-DEL(UA)-EWR-PDX, checking in with Jet Airways, I get the non-elite long haul allowance of 1 bag @23kg. If I return BLR(LH)-FRA(UA)-SFO-PDX, checking in with Lufthansa, I get the Star Alliance Gold benefit of 2 bags @23kg.

Edit - jsloan’s answwr that you’ll likely be charged the lesser of the UA or Aeromar fees is more accurate. A downline carrier is free to charge their fee if it’s less than the FMC. They cannot charge more fees, nor apply a lower free allowance, to the FMC’s non-elite rules.