clothing - What is considered a four-cornered garment?

The commandment to attach tzitzit to the four corners of our garments is a Torah obligation, as per Bamidbar 15:38 and Devarim 22:12.

Our Sages of blessed memory ruled that a garment that does not have four corners is exempt from the obligation of tzitzit.

Later, when the four-cornered garment started becoming a less-popular mode of clothing, Jewish males began wearing a special four-cornered garment known as the tallit katan in which to attach tzitzit to, so the mitzvah would not go extinct.

My question is thus: what is considered a four-cornered garment? I assume something that has four corners along the bottom, but what about dress shirts? Technically speaking, they have two corners at the bottom hem, two corners at each cuff and two corners at the collar. Why are those corners on the cuffs and collar exempt from tzitzit?

Answer

The Mishna Brura on Orach Chayim 10:12 writes regarding this question the following

(Source provided by Sefaria.org)

לו) הואיל ואין וכו' - דהנה הב"י כתב יש לדקדק במלבושים שלנו שיש להם ב' כנפות סמוך לצואר וב' כנפות סמוך לארץ למה אין מטילין בהם ציצית ותירץ ע"ז כמה תירוצים ודחה אותם הד"מ ואמר הוא טעם להמנהג דמאחר דמצות ציצית הוא לכתחלה שיחזיר ב' לפניו וב' לאחריו ומלבושים שלנו הם כל הד' כנפות לפניו וא"א להלבישן בענין אחר אין חייבין בציצית וזהו מה שכתב הואיל ואין כנפיהם עשויין וכו

The Beis Yosef writes that there is to point out that why don't we put Tzitzis our clothes which have two corners near the neck (i.e. the collar) and two corners near the ground and provides several answers for this and which the Darchei Moshe rejects and explains the reason for the practice is since the Mitzva of Tzitzis is ideal when one has two (tzitzis) in front of him and two behind him and our clothes all four of the corners are in front of the individual and it is impossible for him to wear it in another manner (therefore) it is not obligated in having tzitzis on it and this is what he(the Rema [ibid.]) wrote "Since the corners are not made etc. (in a manner which has two tzitzis in the front and two in the back)"

Hope this is insightful