Most steel structure failures happen at connections....where a beam connects to a column, where a support beam connects to a beam, where a tie bar connects to a beam. The Structural Engineer must design the design the steel members and give guidelines for the connections. Many people in the Construction Industry don't understand, though, that the Structural Engineer rarely designs the connections.
A beam bolted to a column with clip angles along the beam web likely creates a pin connection. This means that the beam shouldn't be able to move up or down, nor in or out, but it can rotate a bit. A steel column bolted to a concrete pier with four anchor bolts also typically creates a pin connection. Again the steel column won't go up, down or sideways, but it may be able to rotate a bit.
The fixed connection must stop that ability to rotate. So for a beam to have a fixed connection to a column, along with clip angles, there may be a plate on the top and bottom flanges of the beam that gets welded to the column. With all that welding, the beam can no longer rotate. If a steel column is buried four feet deep in a concrete pier, it also would not be rotating at the point that it exits from the concrete. So those are a couple of ways to create fixed connections.