Again, you are asking beginner questions about building things that are definately not beginner projects. If you would describe what you are doing and why you need to build it yourself you might get more help.
BEC is a term used mostly in RC modeling. It is really just a step down DC to DC converter. To handle converting from 48v to 5v/100A you need a switch mode DC to DC buck converter. At 5 volts and 100 amps, you are talking about a 500 watt converter. That is somewhat more power than the typical power supply in a personal computer, so if you have ever seen a PC power supply that is the size and complexity you are talking about.
This app note describes some TI chips for switching power supplies and the last schematic is exactly what you say you want, a 48v to 5v/100A converter. You can see how complex it is, and the odds of a beginner building something like that and getting it working are very slim.