The only app I miss from Android is BirthDayDroid. This app takes birthdays information from contacts and send notifications about them. Also provides convenient interface too look at the upcoming birthdays. GNU/Linux phones also have contacts and also support birthday fields. But is there a similar app to notify about birthdays?
NextCloud automatically creates the birthday calendar based on Contacts information. So NextCloud imports the events, on the backend.
Any calendar that has alerts should work.