iMac - PCIe slot - yes / no

All of the 2012 ~ 2019 27” iMacs have both a SATA HDD or SSD (if you replace the HDD) as well as a custom blade drive interface. Here is a good reference which is more detailed on the interface which is different through the years The Ultimate Guide to Apple’s Proprietary SSDs

So you can get a real Apple SSD or I recommend going with the OWC pin compatible drives, I don’t recommend M.2 SSDs as much as I love them the addition of a converter makes them unreliable! There are other signal lines the Apple has that are not accessed by the M.2 SSD. Here’s the drive kit iMac Intel 27" (Late 2013-Mid 2015) Blade SSD Upgrade Bundle and the guide Remplacement du SSD blade dans l'iMac Intel 27" EMC 2639 consider the amount of work to get to it to replace it and your down time.