How did the lunar maria form?
From cooled lava when the Moon was forming.
The Moon would have been somewhat volcanically active when forming, the maria are basaltic fields- which is are cooled lakes of lava (which we mistook for seas originally, also maria is latin for seas).
The reason they look so darker compared to the rest of the surface is due to their iron-rich content, and are an awful lot less reflective owing to why they're so much darker.
An annotated map of the Geography of face of the Moon visible to us.
These "seas" cover up to about 16% of the lunar surface, which may sound smaller than it looks, but it is mainly because most of the maria is on the side of the moon that is facing the Earth, with much smaller 'lakes' on the face pointing away.
Hope this helps!