Basic open-source Android apps have felt outdated for years, and the GrapheneOS team is finally stepping in to fix the problem. The developers are shifting resources to completely replace clunky default software with modern tools built from the ground up.
Calendula is FOSS and very good. I liked it so much I donated.
I really liked the UI and UX of Google Calendar and I am amazed at how Calendula offers a near identical experience. Really amazing work!