How collars attach to cassocks
Plus some info on how cassocks work in general
Disclaimer: there is a ton of variation between denominations and time periods, the exact details differ slightly in some styles.
TL;DR: Depending on the priest (or the weather) it'll either be a rabat collar that only extends a few inches under the cassock, or it'll be a neckband collar attached to a shirt under the cassock.
Im focusing mostly on the type that would be worn with a cassock, but there are regular shirt styles too.
Option 1: A Rabat Collar.
Snaps or buttons to hold it shut in the back (and in some options, to hold it to the collar of the cassock). The one pictured also has snaps to keep the front of the cassock flush to it.
Undershirt optional it seems, because [this bishop] doesn't seem to be wearing one with his.
They also come in a variety of lengths, the shortest go under a cassock, but the longer ones can go under sweaters, suit jackets, etc.
Option 2: A Neckband Shirt
These things have so much variety and so many buttons lmao.
Some of them have the black part of the roman collar built into the shirt, others have it removable entirely.
The removable ones have buttons in the back and front that attach the collarette to the shirt, these buttons usually get hidden under the outermost layer of fabric.
These shirts can be worn by themselves in black, but if they're going under a cassock it's usually white. French cuffs are common on the white ones.
Option 3: Directly Attached?
I didn't find anything specific about this but looking at the screenshot at the very top, it appears some cassocks have button holes to attach the collar band directly. ¯\_(ツ)_/¯
Typically fitted like a coat in the upper half with enough extra fabric in the skirt to move freely. Using Father David Michael here because he's the first priest I could think of who wears a cassock regularly. Also, based on being able to see down his sleeve on the far left, he's wearing a rabat collar or similar.
The cincture band (called a fascia, or sometimes girdle) is two pieces: a band around the waist and the extra fabric draped over it. This one closes with velcro.
Cassocks traditionally have 33 buttons down the front, symbolic of Jesus's 33 years of life, and are supposed to be fully buttoned all the way down. (I wear mine wrong irl, only buttoned on the upper half because I cannot be assed to get them all lmao) There are 5 buttons on the cuff, symbolic of Jesus's five wounds on the cross (each hand and foot, plus the side wound). [x]
Colors: priests are all black, Monsignors have purple (magenta in my opinion) piping and buttons, and Cardinals have red piping and buttons or a fully red cassock in formal situations.
And yes, they are definitely supposed to be wearing pants under there. Unless... 😏?