Issues with Picture sizes including thumbnails

I’m finding it hard to work out how these pictures work in WordPress. As I get my product pictures from various sources that have different resolution and pixels. If you have a look at my site you see that the thumbnails are from tiny to massive. I would have thought WordPress would just fit what ever you uploaded into a certain size box. You know just scale to fit box or something. This is not yet a live site, so have a look here. If you have any idea what I’m doing wrong could you let me know.

https://woowaternew.sh0p.co.uk/product-category/free-standing-chiller/

I have tried reducing the pixel size of the large pictures by half hoping the size would reduce, but they stay massive with lower quality. That’s another thing, can someone tell me how word press knows what size to display the pictures as its not done by looking at the pixels.

Thanks