This is based on what I remember from DVD extras I probably haven't seen in close to 20 years so some details might be off. For the most part they reused the Sealab 2020 animations but not the full episodes. Since 2020 was animated on cells, the mouth movements were most likely on their own layers, so 2021 could lip sync to their own lines.

As 2021 went on they did add more of their own animation. I remember someone involved mentioning in early episodes if a character needed to turn around they'd just flip the cells without having any frames in between. Later on they were able to make the frames of the character rotating to make it look more natural.