How did fossify data moved across phones ?

[Happyclem]

I broke my phone (A) and had to change it recently.
I use a lot of fossify app, notably calendar and clock.

Waiting for the new phone (C) to arrive, I used a spare one (B), with fossify app also, but no data in them.

Upon arrival of the new phone (C) I did the "transfer data" thing from phone B to phone C (so not my old main one)

Today, I installed fossify app on my new phone (C) and without importing any save file, my calendar was filed, my clocks and alarms were all their with the name.

Most shocking is that the last timer I used on my old main phone (A) before it died, was expired on my new phone also !

I install all fossify app with droidify (F-droid client).

So I'm very curious now, how did this happened ?
The only thing in common with both A and C phone is the google account linked to it. Fossify is present on the playstore, but that is not where I downloaded them.
Could it be that playstore have some "auto save data in cloud" like in other app, and detected my fossify apps ?

I'm truly baffled because I checked every event that I recently added from phone A, phone B had no calendar event because I was using it for a week or so, so I didn't bother, and now on phone C all fossify app are filled with upmost recent data.

If you have any questions to try to clarify I'll gladly help.

Phone A : Xiaomi Redmi note 13
Phone B : Xiaomi Redmi note 8
Phone C : Xiaomi Redmi note 14 pro+ 5G

[Nikon6302]

I readed some issues in Fossify Contact and heard two solutions:

1. Nextcloud
2. DecSync CC + Syncthing(-Fork)

source: FossifyOrg/Contacts#469

[Happyclem]

I dont have nextcloud.
I do have syncthing but not DecSync. The last time I exported my contact was a year ago, and I never exported my calendar.
Thanks tough, still a mystery. I don't have clean phones to try stuff.

[bjlockie]

I think all the data is stored under the Android system folder which is copied over bluetooth.
I'm just guessing.