Trans women get ~periods~ because some of the emotional etc. symptoms of PMS/periods are caused by hormone cycling -- which is exactly what they're doing, because the boof or inject estrogen and progesterone every week or month or whatever. These levels naturally rise and fall depending on how recently they've taken it, and affect mood, energy levels, etc. (Might also be part of why they're so unstable in general.) Other aspects, such as "craving for chocolate" or "abdominal cramps" are either psychosomatic, caused by other medical issues, or just straight-up lying on twitter for clout.
It's not a fucking period, though. It's just hormone cycling. Menstruation is when the uterus sheds its lining (plus the egg that was previously ovulated) in the absence of conception, and requires an exceptionally specific cocktail of several different hormones that vary in their levels over the course of a month. If you don't have a uterus, you don't have a period -- even females who don't have a uterus (either through hysterectomy or genetic anomaly) don't get periods. They might still get weird symptoms from hormone fluctuations, but that's not a fucking period. Rate me MATI, but it really fucking pisses me off when troons are like "uwu I'm on my period and it's so awful", when what they mean is that they're a bit moody, and want some chocolate, and pretending to have a hurty tummy. Meanwhile, real women have to deal with messily losing chunks of organ through their vagina every month, and still go to work or manage a family and generally keep on living their lives rather than whining on twitter with their feet up.