Populating disk on new computer

The most reliable and quickest method is to connect the old Mac's Time Machine EHD using a USB cable to the new Mac before you turn on the new Mac. When you turn on the new Mac it will launch Setup Assistant, SA will get to a point where it asks if you are migrating from another Mac. Select YES and follow the on-screen prompts. This works 100% of the time.

Do not connect using Wi-Fi, Ethernet, or any other method than the suggestion above.

I recently migrated from my 2011 iMac's TM EHD to my new 2020 27" iMac per my own instructions above and it worked as I fully expected it would.

No, you want to leave the new system files alone. You can use Time machine and migration assistant, though.

Yes, I always use Setup Assistant to migrate. The reason being is if you use Migration Assistant, MA will create a new account whereas the SA method does not, so when you log into the new computer the first time it looks very similar to the old system.

You can move over your files, settings, etc. but you cannot "clone" the old one because Macs cannot boot from any system older than what they came with. So, if your new one comes with 10.15.6, your old one would need to be running that same version in order to clone it. But, when you get a new machine, it is best to use the new OS with it. And, Time Machine does not create clones - it makes backups.