Sync strategy of Anki

Hi all,
This question may be at the limit of the scope of this forum, but let’s try.

I am the author of a free software to manage a bonsaï collection OT2.
This is a small niche but despite its age, it has still some users.
I am considering giving it a reboot and the most important thing it lacks is the possibility of synchronizing the collection between several devices, something Anki had got for years.
What is the strategy/software used at the server side to get this functionality? Is it free? Could it be used to make a custom server to be run on one’s computer to sync between smartphone and computer?

Thanks in advance

I recommend reading this page: Sync Server - Anki Manual

Thanks a lot.