For Google Assistant see Official Google Assistant Integration for openHAB in addition to the official docs. The big thing is you will need Items and need to add one of the supported tags to those Items.
For UI, depending on what you are after look at:
- https://www.openhab.org/docs/tutorial/sitemap.html
- https://www.openhab.org/docs/configuration/sitemaps.html
- https://www.openhab.org/docs/configuration/habpanel.html#designing-dashboard-interfaces-with-habpanel
- [SOLVED] Where to start with HabPanel
Good luck and thanks for posting! Let us know when you run into any problems. We like to help where we can.