wifi speed test drop and not consistence on iPhone 12 and MacBook Pro m1

Your home Network should support the connection of about 25 devices with no slowdowns.

Only when they are ACTIVELY transferring data do they begin to influence each other's throughput (but NOT Transmit rate).

How are you testing the WiFi speed? The way you test (simultaneous downloads over the internet, downloads one at a time, versus sending data between devices, etc.) will affect the results.

This is mostly having to do with how your router is configured, versus an issue with the MacBook Pro. The phone works independently from the Mac, so the Mac itself won't affect the phone speed, but both are going through the router.

Also, are you using iCloud on both devices?

Your Router is perfectly all to communicate with BOTH your iPhone and your Mac with no slowdowns.

If you do the speed test at EXACTLY the same time, they will compete with each other, otherwise NOT.

Be certain you are not using your iPhone as a HotSpot, which will compete with your router for bandwidth in your home.