google sheets – Insert text in every Nth cell

I added a sheet called “Erik Help” and entered the formula into cell D1:


Essentially, I added a virtual column filled with the $C$1 value, then formed a virtual array with this virtual column first, followed by A:B. I used FILTER to get rid of null rows and then used FLATTEN to make a single list.

NOTE: The FLATTEN function is an as-yet-unofficial Google Sheets function.