The bladder sits right behind the pubic bone, inside the pelvic bowl and is well supported by the pelvic floor muscles from below. The urethra goes through the pelvic floor muscles and that’s how pelvic floor muscles are able to influence both these structures.
Mechanically, pelvic floor muscles keep the urethra closed by creating a tension around it. This tension in the pelvic floor, at rest, supports the bladder from below and allows it to quietly fill to the max capacity. When completely full and when socially appropriate to pee, the pelvic floor muscles must relax to allow the outflow of the urine through the urethra.