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 ...
DXA indeed offers out-of-the box support for Navigation based on Taxonomies and has a KeywordModel for mapping a Keyword field to a View Model.
For all other Taxonomy-related functionality, you should indeed use the applicable CD APIs. That is: CIL for 8.x or PCA for 9.x.
If the rich text formatting options are the same across Schemas (or "schema," if @DomCronin is following) you could consider embedding the same rich text format area field and its XSLT filter options where needed.
You would still need to use this field in your various schemas , but changes would be centralized and managed within a single field.