Best Way to transfer Data between a Mobile and a Desktop Application

Hello folks,
i have a desktop application to store various data in a database (up to 10k datasets).
Then i have multiple mobile devices (Android and Apple) to collect data by user-input (up to 2k datasets).

Assuming a not existing wlan or internet connection: what is the best way to transfer or synchronize data between Desktop - Mobile and vice versa ?
I would try to copy the db-file to the mobile device and update the mobile-db via script comparation (slow..).
A) How to access usb-connected mobile storage via desktop-app?
B) How to find, read and write those files on mobile?

Assuming i have wlan if i come home with mobile and dont wanna mess with usb.
C) Transfer data via multiplayer-communication-stuff? Like the two running apps communicate via chat-like system. One dataset per message; then parsed to desktop-db. ... seems also slow.

Best solution would be.. both ^^ the user should be able to choose.
What do u folks think ?

 

It is hard to answer what the "best way" of anything is, because it depends on the use case and the skill set of the user.

My generic answer though is to have a dedicated server on the internet and an account that both the mobile and desktop can log into. Data is transferred between the two devices through the dedicated server. The reason it is in my opinion the "best way" is because it requires no knowledge of networking on the user's part or any skill other than what the user is already used to doing on a regular basis, and doesn't even require the two devices to be in physical proximity because it will just work anywhere.

On the other hand, a USB connection would probably require the user to enable a developer USB setting on the phone. Connecting from the phone directly to the desktop over wifi can involve issues of network topology, software firewall settings on the desktop, or access problems from the phone to the local wifi network.