The DTG printing process involves printing directly to textiles or clothing with a modified inkjet printer that is specifically designed to print garments. It requires a specialized platen and inks that are formulated specifically for cloth textile printing. These inks are printed directly to the fabric, unlike dye sublimation textile printing or heat transfer printing which uses a paper carrier which transfers the dye image using a combination of heat and pressure.
The basic technology used to build a DTG printer is the same technology used to build an inkjet printer similar to those used in homes and offices worldwide, except they cost a lot more, sometimes a lot more, depending on the type of output the printer will produce. The main reason DTG printing t-shirt designs was developed was to create a way to print small quantities of shirts without the cost of having to set up multiple screens to print just a few shirts or a few dozen shirts. You could actually print a single shirt with this technology.
One of the reasons for DTG printing t-shirt designs, though, is that cotton and other natural-fibre cloth fabrics cannot be dye sublimation printed, due to the porosity of the fibers. Poly fabrics like polyester and nylon can be printed with dyes during the heat transfer from the transfer paper to the fabric because they are closed fibers that open up and encase the dye then close again as they cool. Natural fibers are not able to accomplish this, so inks were invented that would fill the gap, so to speak, using inkjet printing technology.
Natural fabrics have been printed for decades using inks that were compatible with cotton, but with the advent of dye sublimation printing, it became the challenge to create the inkjet printers that could print cotton and other natural fibers with similar results, although, in my opinion, the colours don't pop as well on the natural fibers, possibly because they are natural fibers.