Bring comma-separated values in a field into an array

This must be straightforward but I just can’t get my head around it!

I have a field containing comma-separated values (eg. 2,5,11,17) which I want to turn into an array for easy manipulation before then turning back to a comma-separated value to write back to the database.

I’m looking into split, repeats, arrays, array->add value, etc. but getting nowhere!

Can anyone point me in the right direction, please?