When it comes to looking at printing t-shirts by DTG, do the following. First, double check the art and see that it will translate well with the final product being made. Then, optimize and see the final print is as close as it is initially supposed to be. The t-shirt is then covered with a clear solution that allows the ink to be printed on darker fabrics before being heated to stick on top before printing. Like a regular printer, the image is simply printed out like a painter. Then, the shirt is dried by heat to solidify the ink.
A major difference noted already is the economics of it. A print shop that has a manual screen print are spending a few thousands more on a new one and upkeep with ink, tools, and cleanup. For mass production, DTG is the way to go and it is made for the consistently evolution of our technology. But it is too simple of a process and some designs are multiplayer where doing it by hand from the screen printing. So, some prefer the expressive multiple colored design that is stenciled in and carefully traced over before the inks put on the lay its mark. Good DTG printers, on the other hand, costs at least $100,000 and can print white ink on dark cloths, a very difficult thing to do.
There are many ways to put a design on a t-shirt, but it is up to the designer/seller on how it works best for them. The budget, number of shirts, colors, the shirt material, and other factors play a part in screen printing versus DTG. These designs are central to a customized t-shirt because it is what attracts people to it. Deciding how it will be printed will alb up to you and what you feel is right based on the pros of each style.