See the below pictures. Would anyone know how to fix / solve these type of stains?

I assume they are moisture issues - for example, the first spot was due to a wet cushion I sat down for a few days under my covered patio. It's been like that for months. The other areas are more exposed to the elements.

Concrete slab is 5+ years old. In Houston.

It also looks dirty to me. Have you tried power washing it?

IF this is efflorescence then pretty EZ to solve but not always permanently.
Even a non-spec/tract house slab will present such upon weather & wet covers (as u mentioned).
Some go away in time w/o treatment.

https://www.nitterhousemasonry.com/blog/what-is-efflorescence/