As you can see @Nuno shared the official release notes of sdl documentation for good source.
In High level listed out main functional and technical features of sites 9.0 and 9.1
Pre-defined Regions support
Usability improvements for Component Presentations
Blended marketing and product information from SDL Tridion Docs
For your Drop down list component
Schema: add "ddlist" a multi-value text field based on the drop down list category
Component: select all the keywords you want for your dropdown list in the component multivalue text field "ddlist" in the order you want them in the drop down
update your model to add <List>KeywordModel ...