I would like to add that a suppressor can render certain specialized firearms nearly silent if they are used in conjunction with subsonic ammunition. A suppressor can deaden the sound of the initial explosion, but a supersonic bullet will continue to create a sonic boom as it flies through the air. A subsonic round doesn’t create a sonic boom and as a result nearly all of the sound of firing comes from the initial explosion. If that explosion is well sealed and is funneled through the right supressor, nearly silent operation can be achieved. A good example of this is the Welrod used during World War Two, which was quiter than an airsoft gun and was only really audible at point blank range.
TLDR, how quiet a gun gets with a suppressor is determined by the ammunition, the type of firearm, and type of suppressor. Suppressed gunfire can range from as loud or louder than a nail gun to as quiet as a sneeze.
Worth noting that while a nail gun is pretty loud (if you’ve ever been around them without hearing protection), it’s still nothing on being near an unsuppressed gunshot. If you’ve never been up close when even a pistol is shot, it’s much louder then you’re imagining, and louder still than you’ve just adjusted your imagination to. Rifles are louder again.
It’s worth noting that nearly none of the people you see testing suppressors online use a pistol with a slide that is locked so it doesn’t cycle, shooting through a suppressor that has the right type of “lube” applied, with rubber wipes at the very end to let the bullet through then reseal the suppressor for a few shots, before they’re completely shot out.
You don’t get a lot of silent shots, and you’ve got to rack the slide yourself for each of them, but they do get quite a bit quieter than the suppressor mythbusters think they do, running their dry, open suppressors in semi auto.
