Larger datacenters mostly use fiber lines, predominantly running Ethernet already. The fibers uplink to an ISP, carrier or directly to an IX.
ISP uplinks are often terminated by an ISP router, very similar to domestic Internet connections. On the WAN line they use various protocols including xDSL, WLL, (Metro) Ethernet. What they use is of little consequence; the customer usually gets a routed Ethernet connection (handover port). This variant is very similar to a domestic Internet access, just using more professional hardware.
Carrier and IX uplinks are mostly Ethernet and you supply your own router. The handover is direct on the WAN fiber, esp. 1000BASE-LX, -BX, 10GBASE-LR and so on, that runs into your router (or switch) using an appropriate SFP transceiver.