Pull Ups are usually advertised as being like underwear and they are somewhat like underwear because the child can pull them up and down without the parents assistance so they can use the toilet for pee and poop when needed.
The Pull Ups are absorbent and used to catch urine and poop just in case the potty training child is unable to get to the toilet in time or forgets to go to the toilet.
Pull Ups are not meant to absorb a lot of urine unlike regular baby diapers are because when a child is actually in regular baby diapers and not potty trained they are meant to use the diapers for peeing and pooping so the diaper is made more absorbent to absorb the heavy wetting of the child so they don't leak out of the diaper and get their clothes wet.
The Pull Up is not made to absorb as much because the child is supposed to be using the toilet or potty chair to pee in.
But in reality yes Pull Ups are basically just diapers that are more expensive but are a pull on diaper.