Mount SMB

MoldyTaint I see what you're going for, but unfortunately I don't think there's an easy fix. I'm pretty sure that the use of the default file picker is up to the app itself. And I don't believe Android supports creating persistent Samba mounts without the use of a full-fledged Linux environment, like that provided by Termux.

An easier solution might be to use a file sync tool, such as Syncthing. I use Syncthing to keep all of my documents, notes, wallpaper and other files synced between several phones, workstations and a file server. It can just as easily be used to sync a few files between two hosts. Might not be quite what you're looking for, but it may provide the same result.

MoldyTaint
Yes, that's right, even for a PWA (e.g. ProtonDrive).
But with Syncthing it didn't worked for me like expected.(maybe i'm doing something wrong)

I'm using a P7P with GOS, no worries.

Schiller I recall Syncthing was a little confusing when I first used it too, mostly because it was unlike anything I had used before.

Essentially, you'll install the client on each of your hosts. Syncthing will provide an example share automatically, such as Camera Roll, just to give you something to start with. You can ignore this default share for now, or delete it entirely if it doesn't suit your needs.

Once Syncthing is installed on each host, you'll need to add one of your hosts (referred to as a Remote Device) to the other, so they can become aware of each other and sync files between each other. It's usually easiest to add a PC host to a phone host instead of the other way around, since the phone has a camera and can easily capture the QR code of the PC's Syncthing ID.

1. On the PC, open Syncthing and go to Show ID (this is under the Actions menu on Linux, but Windows may be slightly different).

2. On the phone, open Syncthing, go to the Devices tab and then click the plus symbol (+). To the right of Device ID, you'll see a QR symbol. Click that icon, grant the camera permission to take a picture and then capture the QR code shown on the PC. Give this new remote device a unique name (e.g., your PC's hostname, or whatever will help you know which host it is) and complete the steps to add your PC as a trusted device on your phone.

3. On the PC, you should now see a prompt that a new remote device, your phone, would like to connect to your PC. Verify that the ID number belongs to your phone (Open Syncthing on your phone, go to the menu and select Show Device ID), and then accept the connection to add your phone as a trusted device. Give it a unique name and complete the steps to add your phone as a trusted device.

You'll now have two devices that have mutual trust of each other, but not yet sharing files. At this point, you can create a share (referred to as Folders in Syncthing) through the application's interface on either device. During the creation process and at any time after, you can toggle which remote devices should participate in the share. Those devices will then receive a prompt to accept the shared folder; you'll be prompted to choose where those newly shared files should reside on the device.

For the sake of simplicity, I keep most of my synced shares within a root folder on my phone called "Sync", with sub-folders for each individual share (Docs, Notes, Wallpapers, etc.).

There's a lot of good documentation on the official website which will go over best practices, etc. but hopefully this gives you an idea of how to get started.