How can I use an external SSD instead of the built in HDD on a Mac Mini Late 2014?
The two most common utilities for this are Bombich Software's Carbon Copy Cloner, and ShirtPocket's SuperDuper. Both will allow you to create a bootable clone of your current OS on an external volume; then at next startup, hold down the Option key and choose the external volume.
Note that model Mac mini only supports USB 3.0, so while it'll be faster than the internal drive, you won't get the full performance from a Samsung SSD that supports USB 3.1 or greater.