Reverse a string by chunks

Jelly, 2 bytes

sṚ

A full program that prints the result.

Try it online!

How?

sṚ - Main link: string, number                                   e.g. 'abcdefg', 3
s  - split string into chunks of length number (keeping any overflow) ["abc","def","g"]
 Ṛ - reverse the resulting list                                       ["g","def","abc"]
   - implicit print                                                   gdefabc

Python 3, 35 bytes

f=lambda s,n:s and f(s[n:],n)+s[:n]

Try it online!

05AB1E, 5 4 3 bytes

-1 thanks to Dennis
-1 thanks to carusocomputing

ôRJ

Try it online!

     # Implicit: push input
 ô   # Split in pieces of length b
  RJ # Reverse and join