Dedicated servers are relatively easy: Find one of a tons of data center hosts out there, sign up, get your credentials, and your done. But the next level up, data center, doesn't seem to be so easy. From what I've read on the Stackexchange migration, it seems you buy everything and the datacenter just provides the bandwidth and electricity.
Am I right? Is this how data center's usually work? Or do I have it completely wrong
That's correct. They provide:
It's called Co-Location. It's a lot more expensive, but a lot more flexible if you need a custom hardware solution to suit your needs. There is usually a break-even point where its cheaper to do Co-Location than leasing dozens of servers.
Getting access to your servers is impossible in a dedicated hosting solution, but depending on how much money you're spending and the amount of rack space you're leasing any decent datacentre has good enough security procedures to permit you to sign yourself in and access your own servers 24/7 to do maintenance, install new hardware, upgrades, etc.
