Using your left hand thumb to fret notes is a very useful technique
It can make certain changes much easier to achieve - in particular to get an F chord when playing in the open C, or open A minor, position.
This example is an introduction to the technique using an A minor run down - simply using your thumb to put an F note under an A minor chord ( making Fmaj7 )
NOTE - Your thumb does not have to come right round and fret the note from above.
You just approach the edge of the fingerboard at, roughly, a 45 degree angle
( see pic ) and touch the bottom string, fairly lightly , just below the 1st
fret - this is largely a matter of experimentation to find the right spot to
put your thumb to get a clear note- big hands and strength are not necessary