Normally Explorer does not open automatically when connecting phone/mtp. Do you have Galaxy-software installed?
Maybe setting "Explorer-replacement" will work. But from my experience (badly programmed) MTP does not work good with DO (no refresh/empty on refresh, disconnects, unfinished copy progresses,...). I use WLan or direct USB instead.
The very first time I connected my new phone to laptop, Windows 11 asked me with a popup dialog what I would like to use when connecting this device. Directory Opus was not one of the choices and I don't remember if there was a menu item to find another application.
There isn't an option to make Opus the default for that, at least at the moment.
But you're usually better off using FTP or SFTP over Wifi with Android phones, as MTP tends to be unreliable (whether you use File Explorer or Opus; there are issues in both Windows and Android's MTP stacks that make it a poor choice):
Oh, never tested MTP with Win11, but there must be a driver installed to recognize Galaxy phone.
With WLan I meant using WLan-connection to my network to avoid MTP, as MTP is unusable. I use free CX Explorer, you can store often used network-devices (PC, NAS,...) and copy/move between them on phone.