In the builder the page will appear empty and is shown in desktop device overview by default. If you want to build a page or form specifically for another device type, use the button on the left to switch to tablet or mobile.
To start building your page, press the icon with four squares in the top right corner to open the sections menu. Here you can find all the different content section types that you need to build your form. In most cases you would start off by dragging your Header and Footer section into your page, and start from there.
To build your page with more options, you can choose the different sections under Additional sections, or use different Buttons. You can simply drag and drop whatever section you need.
Please note, all sections can be styled with CSS according to customer branding using the Themes module.