Completely different than what OP is doing, I want to clear up some confusion for anyone reading.
She is doing a natural FET, which means they let her grow her own follicle(s). It is a wonderful option. NO estrogen on cd2, just normal ovarian activity. On a natural cycle she grows her lining from her own estrogen like normal, she ovulates as normal and creates progesterone, the embryos are transferred on day 3 or 5 after ovulation. She develops a corpus luteum that generates progesterone just like in any month. Clinic has to keep very close watch on her and be ready to work around her egg(s). And yes there is always a chance that her egg becomes a baby in addition to the DE embryo transferred at the perfect time. No OB would ever guess she did not get pg the old fashioned way since she will have a functioning corpus luteum on her ultrasounds
Your type of FET was a normal medicated FET cycle, not a natural cycle. Ovulation was totally suppressed by estrace, ovaries remain quiet. Lining comes from supplemental estrogen only, not from ovaries or follicles. There is no ovulation, no estrogen, no corpus luteum, no progesterone created from the body, all is from outside sources. Timing can be controlled by the clinic, depending on when they want to schedule a transfer and initiate progesterone. The only thing omitted from your FET was lupron, but that is commonplace for medicated FETs.
The OP might get a little extra progesterone support (in addition to her natural), it is how some of the gals on the over-40 board do their natural or IUI cycles. Theoretically someone could get pg from a natural FET transfer and take absolutely nothing, letting their ovaries churn out everything needed until placenta takes over.
You truly do not see many natural FET DE cycles done because it is harder for the clinic and patient to monitor constantly for natural ovulation. I have done both a natural FET like hers and a medicated FET like yours.