Having a visible bulge in your pants is generally considered an indication that that person has a penis and thus is treated as though they are male. For transgender men, there is, unfortunately no bulge (unless that transgender man has had metoidioplasty or phalloplasty). Many transgender men (and some nonbinary people) experience gender dysphoria regarding not having a bulge and/or not having a penis.
For some transgender men, getting a packer can help grant gender euphoria. Unfortunately, for some transgender men, having a packer can cause additional gender dysphoria due to it not being attatched.
A packer is a prosthetic that is placed in one's underwear to give the feeling/appearance of having a buldge. A packer can be a rolled up pair of socks, or a highly customized silicone prosthetic. What is used as a packer can vary based on both personal preference and budget. Anyone who wants to use a packer can use one, though they are typically used by transgender men.
Packers are used to alleviate gender dysphoria. For transmasculine individuals who lack the genitalia which would create a buldge, it can be comforting to have something there. Having a buldge can also be a matter of safety for those who are stealth.
When starting out with packing, it is generally advised to start out with something small so that you can get used to having something there. Whether this be a cheap packer, or something makeshift. From there, you can figure out what size you'd be most comfortable with. Everyone is different.
The way your packer stays in place will vary depending on the quality and type of your prosthetic. Some of the more expensive and customized slilicone ones are self-adhesive. Others can be secured using specilized packing underwear. Jockstraps and other harnesses can also be used or modified to be used as a packer.
Think of it like this; you can buy a bottle of shampoo and a bottle of conditioner, or you can buy a bottle of 3-in-one that includes body wash as well. The same goes for packers. Different packers will advertise different functions or combination of functions.
Typically, packers meant to be worn throughout the day are not combined with the stroker or play options. Hard packers look unnatural under clothes, and would interfere with day to day life. Day-to-day soft packers are most often combined with STP devices for daily convenience.