Choose the right Thunderbolt cable
Thunderbolt 1 and 2 use the same plug type and cable and are mutually compatible. It's the same type of connector as a display port for a monitor on an older Mac, but you can recognize the Thunderbolt port on the "lightning symbol", which also appears on the connector on the cable here.
Thunderbolt 3 uses the same cable and connector type as USB-C, but also here you can recognize the Thunderbolt 3 on the "lightning symbol".
If your computer has a Thunderbolt 2 port, you can connect a Thunderbolt 3 device (eg a newer Apollo X interface) using a Thunderbolt 3 USB-C to Thunderbolt 2 adapter, e.g. Apple model and a Thunderbolt 2 cable. You will also need the adapter and a Thunderbolt 2 cable if you have a newer computer with Thunderbolt 3 and need to connect a Thunderbolt 2 device (such as a UA Twin MK2 or an older Apollo rack).