How to get my custom ringtone on to iPhone running iOS 15.4.1?

Please guide me through this process step-by-step. I see so much confusing and conflicting information on this topic that I can't figure it out. I will try to give all the info clearly.

Devices and Files:

iPhone 12 mini running 1OS 15.4.1

MacBook running Big Sur MacOS 11.6.2

Homemade custom ringtone with ".m4r" file extension

Status:

The ringtone file is currently on the MacBook's hard drive, but not on the iPhone.

Goal:

Get the ringtone onto the iPhone and make it be the default ringtone for incoming calls.

The main roadblock I'm experiencing is trying unsuccessfully to get the ringtone onto the iPhone from the MacBook. There is no iTunes in Big Sur 15.4.1 (now it's called the "Music" application), so I can't "sync using iTunes" as many places recommend. I also can't figure out how to "sync with Finder": I have successfully plugged the iPhone into the MacBook with a USB cable, and selected the iPhone in the sidebar, nd had each device "Trust" each other -- but when I select the "music" tab and click the "Sync with iPhone" box and then click "Sync" -- nothing happens. The Finder doesn't recognize the ringtone file as being "music," I guess.

Where on my computer's hard drive do I place the file to have the computer recognize it as "music"? Or - do I need to add the ringtones to the "Music" application? If so - how? Can't figure it out.

Any advice you can provide would be most appreciated. And yes, I've already looked at the support pages Use the Finder to sync your iPhone, iPad, or iPod with your computer and Use tones and ringtones with your iPhone, iPad, or iPod touch to no avail -- the steps they outline don't work to solve my problem.