There are several ways to install radiant heating.
You can bury the PEX tubing into a concrete slab.
You can attach the PEX to the bottom of the sub-floor.
The PEX can be stapled to a wood sub-floor and covered with light weight concrete or mortar.
Once the tubing is in place, the finish floor can be installed. Most types of floors can be accommodated but there are design issues to consider with some.