Some 2 year olds still wear diapers all the time and some 2 year olds may only wear diapers at night for bed wetting.
Most 2 year olds however are still wearing diapers full time and are not potty trained although some kids may be potty trained as early as 18 months but some kids including my own kids took until near 4 years old to become fully potty trained.
So my kids were still wearing diapers full time at 2 years old because they were not potty trained and didn't use the toilet at all.
After 3 they would wear diapers but would go pee in the potty sometimes and didn't poop in the toilet until they were closer to 4 years old.
All kids potty train at different ages and 2 years old is still pretty young so most 2 year olds you see today are likely wearing diapers full time and are not potty trained but some 2 year olds may be potty trained and out of diapers.