Why do some parts disappear and others dont when i weld shapes?

Why when i weld all these shapes together, it will remove the bigger section in the middle but instead deletes the rectangles at the bottom of the shape instead of removing those areas from the larger shape as well? PS these images should be flip flopped

Have a look at this to understand the different Boolean operation and when to use each:

Weld keeps the overlap of all shapes, which in your case is a single large rectangle. What you want to do with the lower shapes is subtract them, and that’s a different operation, as the link Rick provided explains.