Hi @noor_mohammed! So the ODK-X tools can be used in a variety of different configurations depending on project needs. There are cases that use it with NO internet at all, cases that export csvs and securely transfer those, and cases that use servers.
In terms of data security, in all cases, you always want to make sure your tablets are encrypted and protected with strong unique passwords. Then whatever system of getting data from the tablet to the office team you use, how you set that up (ensuring encryption, secure and unique passwords, etc.) is going to be the most important thing.
I have myself have used server options on some projects and Dropbox + Boxcryptor for other projects. The choice was usually based on which logistics were going to be easier. Both options can be very secure, but it depends on your setup. If you do not encrypt files/transfer, or make a bad password of abc123 for example, this is not secure no matter what setup you use. If you want to set up an ODK-X sync endpoint on a local server, here are the directions: Setup ODK-X Sync Endpoint Manually — ODK-X Docs and you’ll need to go on past this page to setup users, etc.,