I think of sex as mostly binary and gender as pretty much a continuum. There are intersex individuals but not many, and to treat sex as binary has always been a fairly good approximation for practical purposes, Women and Children first when the lifeboats are lowered and it's not a time for argument. But gender characteristics are distributed along a continuum rather than distributed into a binary of trans and cis. Most boys, by whatever are the accidents of their upbringing, acquire some feminine characteristics and most girls some masculine ones. Some children end up more thoroughly transgendered than this but manage to find niches in life which suit them.
There are a very small number of people who feel strongly that their gender character is misaligned with their body and who want to change their body, and some do so using whatever chemical and surgical methods are on offer from the current medical profession. But I doubt that those who change sex in this way end up completely cisgendered in relation to their new body. They will still have gender characteristics left over from the sex they are leaving. It can get very complicated and that’s fine, though complicated does not mean heroic.
That gender is a continuum has always had some recognition. The terms cissy and tomboy indicate very obviously that there was long ago a recognition that some boys had more than their average share of feminine characteristics and girls ditto for masculine. Cissies had a bad press, but tomboys usually a more indulgent one. The differences sometimes had obvious explanations in accidents of parenting and the sex distribution of siblings. An isolated boy growing up with six sisters would probably end up playing their games; a girl growing up on a farm often got handy at doing things that we might think of as very masculine, like killing. And, living on a lighthouse, Grace Darling knew how to row a boat in a storm.
Parents very often try to ensure that their children get gendered in as binary fashion as possible and schools to their shame often co-operate in things like school uniform and separate classes for this or that, swimming or cookery. But children are not docile in these matters and may resist. They are very alert to other forms of fun than the ones they are being channelled towards.