You can change per device (Mobile vs Desktop):

1. Block position, size and alignment (drag/rescale on each view)

2. Section row count

3. Stacking/order of blocks

Changes that affect both (shared across devices):

1. Text and images

2. Links, buttons’ text, and most block content/settings

3. Section backgrounds (image/color/video) and most section settings except row count

No - Squarespace sites are responsive by design. Instead, fine-tune your Mobile view by changing block sizing/layouts.