[Moose408]

Your understanding of picking down on the downbeat and up on the upbeat is correct. Upbeats typically implies eighth notes, if you are just playing quarter notes then there is just a downbeat.

Another way of saying it that a measure in 4/4 time you would count it

1 and 2 and 3 and 4 and

The numbers are the downbeat and the “and” is the upbeat. If there is no note being played on the upbeat, then you would just strum with all down strums.