The University of Drilling is a practical learning site, not an accredited school.
A hole is made to a geologic depth at a location using a drilling rig with equipment at the surface used to lift and operate various downhole tools including the the drilling string, which rotates and pumps fluids down the string to removed bored cuttings.
Drilling mud is used to establish well control, transport cuttings to the surface, provides fluid loss control, lubricates the string and cools the bottom hole assembly until casing can be hoisted into the borehole where it is cemented.
Because the fluids from various permeable formations cannot mixed with the fluids of others, e.g. ground water used for drinking and farming, casings are placed and cemented to isolate one permeable formation from another.
Consequently, equipment must pass through he previous casing string diameter and the borehole diameter becomes smaller each time a casing string is installed.
Once the various sizes of bores and casings have been placed a well completion is installed to isolate pressures and fluids, and then stimulate or restrain the formations so that produced fluids, typically hydrocarbons, can be produced with minimum amounts of extraneous fluids.
A well comprise various hole sizes and casing strings with tubing within the casing used to produced fluids from a desired portion of the well.