What is the safety guarantee using the QGIS platform for data storage? I want no data loss and easy access to data when stored. Can passwords be used to safeguard data and how do I give permissions to users?
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From the user guide: "...there is no warranty for this free software."
As QGIS is a user application, you should make sure you have appropriate backups. You can use external applications (e.g. archive / "zip") applications for encryption and backup, and the security of the result depends on those applications, not on QGIS.
As an addition to BradHards answer that there are no tools for backup/permissions/secuirty from the Qgis side:
Especially your last point, permissions to users, is a typical scope for databases without local access. The normal usecase is to setup a database that takes care of the user-roles, security and backup and then connect it to your applications via direct connect or a web service.
On the security side, you have many things up to row-level security (that a single user can only see single entries meant for him/her) and overall data encryption.
On the prevention of data loss you have checksums for datapages that will tell you when there was a corruption like a rotten bit.
And on the backup side you have different forms of backup up to versioning, point in time recovery, server replication and more.
That being said with the previous examples apply to Postgres as database(and likely for the other big databases, that i´m not familiar with), which is the standard open source for geodata, you will usually don´t try to achieve this for your local application. Just set up a database when it is important and you want to take care of all these matters together.