what is fastest way to migrate from a mac to another mac

If you have already setup the M1 then connect the 2015 iMac's Time Machine EHD to the new iMac using USB cable ( you will need an inexpensive adapter available from Amazon for less than $10) and use the instructions I provided in the link. If you have not received or setup the new iMac yet then make the same connection using the 205's TM backup and then startup the new iMac. Eventually Setup Assistant will ask if you are migating from another Mac, select yes and follow the on-screen directions.

If for some reason the 2015 does not have a Time Machine (or a bootable clone) backup, then buy and EHD such as a OWC Mercury Elite Pro and create a backup using Time Machine ( Back up your Mac with Time Machine - Apple Support ) and then migrate. You will need to be backing up the new computer and can re-use the EHD you bought on the new computer.

The fastest, easiest and most reliable is to use the instructions in Setup Assistant and Migration Assistant

I am not sure what you are attempting to communicate, can you elaborate?

Your welcome and good luck. Depending on the amount of data on the old Machine the migration can take anywhere from 1-3 hours.