You say that your Mac is an iMac (Retina 4K, 21.5-inch, 2019).
iMac (Retina 4K, 21.5-inch, 2019) - Technical Specifications - Apple Support
MacTracker indicates that Apple sold those Macs with
- A 1 TB 5400-rpm SATA hard drive,
- A 1 TB Fusion Drive (which, according to Wikipedia, would have had only 32 GB of SSD storage)
- 256 GB, 512 GB, or 1 TB of flash storage
If you are looking to speed up the Mac, and you are currently running off an internal hard drive or Fusion Drive, you may want to look into getting and booting from an external SSD.
That Mac has four USB-A (USB 3.0) ports and two USB-C (Thunderbolt 3) ports, so it can support just about any of the external SSDs currently on the market. And it wouldn't have to be a Thunderbolt 3 / NVMe SSD to deliver quite an improvement over a hard drive.