Keep in mind there are 2 basic ways we hear.
The first is sound waves through the air (consider this our "normal" hearing) aka "air conduction" or AC, so anything that prevents the sound waves from getting to the TM (tympanic membrane, aka ear drum) will help. Think ear plugs or muffs.
The second is sound waves conducted through the bone, aka "bone conduction" or BC. This is essentially through the mastoid process, that bony prominence (knot) you can feel behind and just below each ear. This is how you hear underwater while wearing a wetsuit hoodie or how you "hear" concussive forces. Plugs won't help here, only muffs that cover the mastoid process will help.
So, plugs help limit AC and muffs help limit both AC and BC. "Double protection" or wearing plugs AND muffs help by further limiting the AC (air conduction).
The type of hearing protection needed/desired will depend on several things - environment (outside v inside), comfort, fit, ancillary capabilities (electronic input for radios), and price.
Following is only my experience, your's may vary.
Outside - foam plugs are sufficient for handguns
Inside - plugs AND electronic muffs
If I'm doing training all day, outside, I wear MSA Sordin Supreme X with the neckband (allows me to wear a boonie hat over without problems) https://www.amazon.com/MSA-Sordin-Supreme-green-cups/dp/B010E12YNK/ref=sr_1_9?crid=1C02U9O4X8CPX&dchild=1&keywords=msa+sordin+supreme+prox&qid=1588780955&sprefix=MSA+Sordin%2Caps%2C160&sr=8-9
I used HL (Howard Leights) for years doing NRA classes but they always gave me a headache after a couple of hours from pushing on my eyeglasses arms. Simple fix - buy the gel-earcups (instead of air/foam), MUCH more comfortable though still give me headaches after ~ 4 hours constant use.