An iMac (Retina 4K, 21.5-inch, 2019) would have come with
- A slow 1 TB, 5400 rpm mechanical hard drive
- A slow 1 TB Fusion Drive that had only a small amount of SSD storage
- 256, 512, or 1 TB, of flash storage (i.e.., a SSD)
If your Mac's drive falls into one of the first two categories, you probably would benefit from an external SSD.
Your Mac has both USB-A (USB 3.0) and USB-C (USB 3.1 Gen 2, Thunderbolt 3) ports, and so can use:
- USB 3.0 / SATA SSDs
- USB 3.1 Gen 2 / NVMe SSDs (attach these to one of your USB-C ports for best performance)
- Thunderbolt 3 SSDs
It is possible to upgrade the internal drive, but opening up, disassembling, and reassembling that iMac is a tricky job best left to a repair shop. And you might only be able to install a SATA SSD – not a faster NVMe circuit board "stick" one. When I looked on Other World Computing's site, they said that their stick ID wasn't compatible with your iMac. An internally-installed SATA SSD would not be much faster than a SATA SSD in a USB 3.0 enclosure, and it would be slower than external NVMe SSDs in USB 3.1 Gen 2 or Thunderbolt 3 enclosures.
All and all, if your iMac's internal drive isn't actually failing and crashing the Mac, but is just slow, an external SSD is the way to go.
