How does a 3D printer print itself given the huge size?
3D printers obviously do not print themselves. 3D printers are sometimes used to make parts for other 3D printers, and this is what is meant by "self-replicating" 3D printers (which is a bit of a misnomer, considering that all designs require a substantial amount of non-printed "vitamin" parts).

A printer that is able to produce many of its own parts is the RepRap snappy, which (as the name suggests) consists of plastic parts that - while they each fit within the build volume - snap together to make up the larger parts for the printer.