Is MAC OS free to download

Yes, for many years no acOS upgrades have be no cost.

The following Mac can be upgraded:

MacBook introduced in 2016 or later

MacBook Air introduced in 2015 or later

MacBook Pro introduced in 2015 or later

Mac mini introduced in 2014 or later

iMac introduced in late 2015 or later

iMac Pro introduced in 2017 or later

Mac Pro introduced in 2013 or later

View the complete list of compatible computers.