@padams35 is correct.
If you decide to go with a 2.5" SATA SSD, then the Crucial MX500 series is a good option. The OWC Mercury Extreme 6G is also another option. Avoid the Crucial BX500 series since it is absolute junk. I would also avoid the OWC Mercury Electra 6G SSD as well since I didn't care for the last one I encountered....the Extreme version is a higher quality SSD and should be better although I have never used it.
If you decide to go for a blade style SSD, then the OWC Aura SSD is the easiest option since it has the same proprietary connector that Apple OEM SSDs use so it is a drop in replacement. Plus if your Mini did not ship with the blade style SSD, then you will also need a flex cable to attach the SSD as well as a mounting bracket. If you wish to go with an M.2 style blade SSD, then you also need to use a third party SSD adapter.....Sintech is the best option since I have never seen anyone have any complaints about it, but I have seen people report issues with nearly every other adapter.