As for Nemo, give it a try:
Install Nemo and execute:

xdg-mime default nemo.desktop inode/directory application/x-gnome-saved-search
gsettings set org.gnome.desktop.background show-desktop-icons false
gsettings set org.nemo.desktop show-desktop-icons true

Check: xdg-mime query default inode/directory

    pomon Hey there, I just tried these prompts, but in he last one the terminal returns:

    No such schema “org.nemo.desktop”

    When I enter xdg-mime query default inode/directory it keeps returning:

    org.gnome.Nautilus.desktop

    Staudey No, I was using the default Plata Noir Compact, but then I changed to Adwaita to test if it was something missing in the theme itself, no success, so I changed it back to Plata Noir Compact

    pomon Hey, I managed to make it recognize nemo. All the prompts worked now and it returns nemo when prompted xdg-mime query default inode/directory but the issue still persists. I can't download anything nor upload anything.

    Guys, please help me. I can't upload files through Telegram nor Whatsapp apps nor download or upload through Brave or Firefox.

    I already installed Nemo, and did what @pomon suggested, but it's not workin

    when you navigate to find a file, select a file, then click OPEN on the file in order to upload--isn't Engrampa the middle man? not nemo/naut?

    PS--I would get rid of every shred of nautilus including all folders in /home.
    I know I'm missing something here

    You need to write more. What exactly is going on. Open Nemo in the terminal and see the errors. What does it mean that you can not send via Telegram.... others? Review the logs (journalctl).

    For instance, when I open Gmail to download the files sent by my coworkers, if I click "download" nothing happens. It was supposed to pop a window that allowed me to choose the file name and path.
    When using Notion if I press on upload image and nothing happens, it is supposed to pop a window that allows me to browse my files and select the file to upload.
    When using Telegram, the same thing, when I right-click a file and select the "save as" option, it doesn't pop the window that allows me to browse my files and choose the file name and path.
    If if press on the upload button, same thing, the file selection window doesn't pop.
    Same thing on Whatsapp and anything I try to do on Brave(my browser)
    When I drag and drop files on Telegram to workaround the upload process, it says "file [name of the file] is empty"

    This all started after past week update :/

    Can you try running the step 1-3 in here. And then sudo usysconf run -f . It is a wild guess, but this sound like something incomplete update.

    Edit: the step 3 might take times without any output. It is normal

      It seems like this was introduced in Gnome 47, as seen in this post.

      https://unix.stackexchange.com/questions/783899/debian-testing-gnome-47-nautilus-doesnt-start-anymore-why

      But I didn't manage to run these commands on Solus:

      sudo echo "GSK_RENDERER=cairo /usr/bin/nautilus \$@" > /usr/local/bin/nautilus
      sudo chmod a+x /usr/local/bin/nautilus

      It says

      bash: /usr/local/bin/nautilus: No such file or directory

        Where do we stand? Are you back to Nautilus or are we still running Nemo? If nemo, create a file in ~/.config/autostart nemo.desktop with the content

        [Desktop Entry]
        Type=Application
        Name=Nemo
        Comment=Start Nemo desktop at log in
        Exec=nemo-desktop
        AutostartCondition=GSettings org.nemo.desktop show-desktop-icons
        X-GNOME-AutoRestart=true
        X-GNOME-Autostart-Delay=2
        NoDisplay=false

        and reboot

        henriiquecampos sudo echo "GSK_RENDERER=cairo /usr/bin/nautilus \$@" > /usr/local/bin/nautilus

        If you want to create such a file in a given location, I suggest echo "GSK_RENDERER=cairo /usr/bin/nautilus \$@" | sudo tee /usr/local/bin/nautilus then chmod...
        Check ls /usr/local/bin

          pomon That worked!

          I uninstalled Nautilus, installed Nemo again and created that desktop entry you suggested, literally copied pasted it.

          It still doesnt work for Telegram and Whatsapp, but thats already some progress.

          Thank you! Could you explain briefly why did it work Im̀ completely unlettered in linux, so you could really help me get some insights for future issues.

            henriiquecampos Could you explain briefly why did it work

            The idea is for the system to load Nemo as the main manager and not look for another one, in this case Nautilus.

            henriiquecampos It still doesnt work for Telegram and Whatsapp,

            I don't really understand what's going on with instant messaging. I know there is Telegram for Linux, but I haven't heard of WhatsApp. What exactly happens when you want to download something from these programs. It's best to run them in a terminal.

              pomon I installed them using flatpak, I don't know how does that actually work, but if flatpak runs through somekind of container, might have something to do with that.

              What happens is: nothing. I press on upload and the program freezes instead of opening the pop up window to select the file. Same for downloading. I will run through terminal using flatpak to see if I can get any meaningful error.

                henriiquecampos, I'm not sure if this will solve your problem, but maybe worth a try?

                I've found often when I have problems with flatpaks accessing local files, it's down to how flatpak sandboxes the local file system, at least as far as I comprehend it. There is a flatpak program available called flatseal which allows the user to easily change flatpak permissions on a case by case basis. It may be worth installing flatseal that allows an easy way to manage problematic flatpak programs. In this case setting permission for giving access to your home folder files. Here is a screen grab of Flatseal adjusting this setting for a flatpak allowing access to the home folder (in this examplle goldendict-ng, but it would be the same for any other flatpak)