Your Mac has an internal bay for a 2.5" SATA hard drive or solid state disk. If it had originally had a Fusion Drive or a SSD, it also has a place where you could install a faster "circuit board stick" SSD.
Other World Computing – OWC Solid State Drives For iMac 21.5-Inch (Late 2013 - 2019)
However, disassembling the Mac in order to replace the drive involves extensive, risky surgery. Since that Mac has both USB-A (USB 3.0) ports and USB-C (USB 3.1 Gen 2, Thunderbolt 3) ports, a better course of action might be to get an external SSD, clone your internal startup drive onto it, then start up the Mac from the external SSD. That wouldn't involve risky surgery or an expensive trip to a repair shop.
These days, you can often get 1 TB external SSDs that attach via some form of USB for under $100 USD.