The Local Calendar integration stores the calendar on the server running Home Assistant, so as long as you can access the server remotely, you should be able to access it through the Home Assistant app. If you want it stored offline on a mobile device, there’s also an integration for calendars stored in a .ics file which you could sync with something like syncthing.
I used to find stuff like this fascinating. Like if collecting my data can help me, why not? But technology has gotten to a point that it’s just straight up creepy how our every single waking moment can be tracked and collected, even if it’s me collecting it. It’s like watching every dystopian sci-fi story come to life in real time.