How would I replace default Windows Explorer with an alternative one, say Q-Dir?

Run regedit.exe, go to [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon] find "shell" string and change value from explorer.exe on Q-dir with full path, then reboot.

There is a program called InstantSheller that can start programs instead of the Windows shell.

It does essentially what Ratin's answer does in the registry, with a few more UI options.

I use it to boot my HTPC's software, hiding the Windows boot logo, login screen and explorer. You can revert the changes by opening the application again and clicking undo.

While it's not a full explorer replacement, I prefer just replacing the launch target of the Win + E shortcut following the approach for Windows 10 / 11 described here. This ensures the normal shell functionality (e.g. Start Menu, task bar, system tray etc. is left untouched)

For my portable version of Q-Dir a registry file looks like this

Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\SOFTWARE\Classes\CLSID\{52205fd8-5dfb-447d-801a-d0b52f2e83e1}]
@=""
[HKEY_CURRENT_USER\SOFTWARE\Classes\CLSID\{52205fd8-5dfb-447d-801a-d0b52f2e83e1}\shell]
[HKEY_CURRENT_USER\SOFTWARE\Classes\CLSID\{52205fd8-5dfb-447d-801a-d0b52f2e83e1}\shell\open]
[HKEY_CURRENT_USER\SOFTWARE\Classes\CLSID\{52205fd8-5dfb-447d-801a-d0b52f2e83e1}\shell\opennewwindow]
[HKEY_CURRENT_USER\SOFTWARE\Classes\CLSID\{52205fd8-5dfb-447d-801a-d0b52f2e83e1}\shell\opennewwindow\command]
@="C:\\Users\\[username]\\Downloads\\Q-DirPortable\\App\\Q-Dir\\Q-Dir_x64.exe"
"DelegateExecute"=""

Replace C:\\Users\\[username]\\Downloads\\Q-DirPortable\\App\\Q-Dir\\Q-Dir_x64.exe with your Q-Dir installation path above (and use \\ to escape \ correctly as shown). Then import the file into the registry (e.g. via double-clicking it in Explorer + confirming the registry import).

After that Q-Dir will launch when Win + E is being pressed instead of Explorer

There is a way! I discovered it recently, steps:

1. Open regedit

2. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Folder\shell\open\command

Inside that key delete DelegateExecute and rename the Default to the proper location of the file manager(here is Dolphin from KDE in my case), like:

"C:\PATH\dolphin-22.12.2-1828\bin\dolphin.exe" %1

• The %1 is important (for the arguments)

This will:

• Evertytime you open a folder it will open with Dolphin at that location

PS: for Dolphin there are more tweaks necessary since you will need to create *.desktop for shortcuts, and change the default location to "default" for the above to work.

Also add @kf06925 answer so everything will be nice and smooth!