WF implements a style cascade that by default starts at the base desktop breakpoint on down. So you need to follow that approach by adding a class to an element and set it’s style on desktop. Then change it at smaller breakpoints.
There are plenty of great instructional resources in the University. Invest time into learning the tool and working with it should become easier.
Depends on what the settings are that you set for an element. An element with fixed width for example would not change at smaller breakpoints whereas one set to a max-width, and a percentage-based width would. Webflow does not do any magic, it lets you enter values visually and those values need to be based on sound practices with understanding of how CSS and HTML work. Take the 101 course and you will have the necessary fundamentals. No pilot flies a plane on the first day.