There are three major categories of potential surgical intervention. For transgender women, First would be a breast augmentation, Second would be facial feminization surgery, Third would be Gender Reassignment. No order is implied in this listing.,
None are >required< in order to transition.
With proper Hormone Replacement Therapy, most trans girls develop some about of breast growth. Whether or not they want a BA is individual preference. Breast are a major indicator of female status, so are highly desired by some. Facial Feminization Surgery helps to correct male bone structures to attain a more female presentation. Think of the typical heavy brow bone that men have, and compare to lack of same in women. The SRS (or whatever acronym you prefer) is not necessary, because how many people are truly aware of what is between someone's legs.
Cost is an issue. BA is probably $6,000 or more. FFS is on the order of $20,000 to $40,000. SRS is surprisingly inexpensive, $15,000 or so. (American pricing, medical tourism can be cheaper).
All this is individual preference, and to some extent luck of the genetic draw. It's a lot easier for someone who's 5'-4" to transition compared to someone who's 6'-0".
Age can be a major factor. It seems that as we age, we become less concerned about what others think of us.
Many of these issues can be addressed with early intervention. Puberty blockers to affect height and bone structures. HRT to develop female characteristics, which is more effective the younger one is.
Nitty Gritty time - I have not had any surgical interventions, although I live my life as female. I had a consult with a famous FFS surgeon and his proposal was in the range of $40,000. I had a consult with a famous SRS surgeon, and after discussion, he agreed that SRS recovery would be problematic due to my short arms. Since I'm 5'-4", size 9-10 shoe, women's petite clothing I pass very easily. Virtually no Adams apple, and was vocally gendered female from my early 20's.
ETA - I can't speak for transgender men, not fully knowing their issues. But I do know that Testosterone HRT can have a major impact on a biological female body.