There are 2 ways to make a modal in webflow. One is using pure interactions and the other is using custom code.
Here’s the custom code option [Tutorial] How to create a modal/pop-up in Webflow - Webflow Tips - Forum | Webflow
The same effect can be done in interactions without the custom code. Tutorial: 99.9% Webflow Created Modal Tutorial/Freebie Clone-able Page - Webflow Tips - Forum | Webflow
Good Luck! Modals are awesome!