is DDR3 RAM similar to DDR3L RAM for 21.5 iMac 2013?

First, "DDR3L" is not specific enough. You need "1600MHz DDR3L SO-DIMM PC12800 204 Pin." 16GB (2x8) is the maximum supported.

Second, 21.3" 2013 iMac memory is not user accessible. That is an extremely difficult upgrade that requires completely disassembling the entire iMac. I'd strongly suggest calling your local Apple Stores and Apple Service Providers to see if any of them can do this upgrade for you.

macs are pretty memory module picky

I would go to crucial.com

and choose your imac model and view what modules setups they have tested with the model