Calendar App: When should i run "occ dav:migrate-calendars"?

I migrated from owncloud 8.2.11 to nextcloud 9.0.58 recently.
As google told me the sync-function is now integrated into nextcloud core and the calendar app is a frontend only.

Does this mean i have to run “php occ dav:migrate-calendar” or is this an optional step?
When do i need to run it and when not?
Do i need to re-configure external clients (Thunderbird, iPhone, Android)?

Same question, but for contacts:
When should i do the following commands?
;# dav:migrate-addressbooks Migrate addressbooks from the contacts app to core

I think normally this should happen automatically when the user accesses his/her calendar and address book. If not all users have logged in or if you want to be sure, you can trigger this conversion manually (and if you install through NC 10 or NC 11 directly). This is especially important for upgrades to NC 10 when the old calendar/contacts-tables will be deleted.

My calendar / contacts app are not at the most up-to-date version. Do i need to update the apps first before i migrate the data to core?

I don’t see calendar/contacts/tasks apps at all. Am I looking in the wrong place?

UPDATE: Figured it out. When I updated all the way to NC 12, I was able to go to apps -> disabled apps and update calendar, contacts and tasks, and then enable them.