I always forget this feature and because I forget it, it makes designing costumes for sprites so clunky for me!

bitmap images are made of pixels and vectors are made by mathematical calculations. hope this helps

Bitmap makes your drawing pixelated, because it is made of pixels. If you import an image, it is likely that it will come up as Bitmap. Bitmap also has a special tool that you do not have in Vector. It can select part of a drawing.
Vector is made by mathematical calculations. Therefore, it has a smoother look, and (personally,) it is easier to use. When you use the select tool in Vector, it selects the whole part of the drawing. Images you import into Scratch will only appear in Vector if:
1. It is a .svg image
2. You convert it to Vector after loading the image.
Hope this helps!

To me, bitmaps are more pixelly and vectors are more popular and better-looking.

Vector is made out of shapes, that you can move separately, even after leaving the costume. Shapes are made out of bezier curves, that you can change with the reshape tool.

Bitmap is made out of pixels. Shapes on a bitmap are saved by overwriting the pixels of the background, so you can't move them after that.
Bitmap lets copying rectangle shaped parts of the image and doing flood fill

Thank you everyone!

Also vectors can only be svg files i think