I guess you did not use a web based calendar, like gmail calendar. Instead you have two local machine based calendars in two different profile folders.

I use Thunderbird local calendars on my computer.

I run the same Thunderbird installation when running Win 10 and Win 11. The email inbox, including many folders I created to save important emails, is the same whether I'm running Win 10 or 11. I can see these folders also on my Yahoo email server. It is just the calendars that are different.

I do have a calendar on Yahoo and I can get email reminders, but there is no "snooze" function. I like the "snooze" function I can get with local calendar reminders.