Secret folder or Proton Vault
On several devices, there are multiple applications to create secret folders, such as on Android there is "Gallery Vault" or the Google vault, or the Samsung secret folder, but the problem with these products is that they are products from big brands, or in other cases, bloatware, or adware, or simply unreliable.
Which is why I suggest creating a Proton Vault, an app to encrypt files in the security of an inaccessible and hidden hidden folder, like, hide it under the mask of a Calculator app, or other ways depending on the user! And with a Proton Drive cloud sync to keep the vault synchronized on multiple devices at once, using Proton Drive storage quota, of course.
Although, Proton Drive already exists, with password-protected folders, and more, why suggest anything else? Well, you can't camouflage or hide Proton Drive completely, right? Anyone who sees the app will know that it is instantly, Of course, you can hide it yourself using third-party applications, but it would make the process somewhat complicated and tedious. Therefore, an app capable of camouflaging itself under a more innocent app that does not reveal that it is a hidden folder/secret vault would be better.
Proton's goal is privacy, right? Well, personal files need to be taken care of too.
PD: I used Google translate to make this.