We wear a timing chip on our ankle, ala this pic. (I am 27)
Close up of one....
They have various timing mats set up.
When you cross a mat the computer starts the time, marks the time, or stops the time.
I believe we had a starting map by the pool. A pool to bike transition one, one coming back off the bike and one for the run. I believe my bike time includes both T1 and T2, but I can't remeber for sure anymore.