Ok to start with I’m going to separate out the OPs question about genetics and sexual characteristics, and the boxers in the olympics.
if you simply define woman as XX and man as XY then yes, women don’t have XY chromosomes. That’s fairly simple biology but doesn’t reflect some of the developmental issues that can arise though. Other posters have spoken at length about this and they are scientifically correct, there are times when XY chromosomes can produce a child that is identified as female, doesn’t go through a male puberty and grow to adulthood without necessarily knowing that they are XY. In some developing countries they get married and only discover issues when they try to have children and are, of course, infertile because they are not producing ova. There can be a variety of androgen insensitivity issues whereby, although XY, the body’s cells either don’t respond at all to androgens (such as testosterone) or respond only slightly. These people can appear very feminine, or more androgynous.
basically human developmental biology is complex and fascinating and not nearly as straightforward as XX - female and XY - male.
turning to the boxers. We have no idea what the medical position of the two boxers concerned is. For whatever reason, they have not chosen to share their intimate medical records, as is their right to do, I suppose.
IF the boxers have not gone through a male puberty and therefore not had the advantage that has on muscle and bone development, then it is right and fair that they compete in the female category. They are functionally female.
IF they have gone through a male puberty, then they have advantages over women and it isn’t safe or fair for them to compete with women.
the difficult case comes if they have partial androgen insensitivity. I suppose each athlete would have to be assessed on an individual basis - how ‘male’ their muscle and bone density is, perhaps. Because I’m not a sports scientist I have no idea if it’s even possible to assess this.
all of this is impossible to accurately judge on looks alone. Women CAN look very masculine, especially if tall and muscular.
in short, a) it depends on how you define what female is, but most biologists will say it’s not always clear cut.
b) it very much depends on the medical history of the athletes concerned. It’s not something we can or should be judging on appearance alone.