To static either a row or a column, put a $ sign in front of it. So if you were to use the formula =AVERAGE($A1,$C1)
and drag it down the entire sheet, A and C would remain static while the 1 would change to the current row
If you're on Windows, you can achieve the same thing by repeatedly pressing F4 while in the formula editing bar. The first F4 press will static both (it will turn A1 into $A$1), then just the row (A$1) then just the column ($A1)
Although technically with the formulas that you have, dragging down for the entirety of the column shouldn't be a problem without putting a $ sign in front of the column. Setting the column as static would only come into play if you're dragging ACROSS columns and want to keep using the same column, and setting the row as static would be for dragging down rows but wanting to use the same row.