The difference between low dose vaginal estrogen and systemic estrogen comes down to concentration and dosing.
Similar to a low dose hydrocortisone cream (1%) which you might apply to a rash, low dose vaginal estrogen (0.01%) does not significantly affect serum levels.
This means low dose vaginal estrogen is safe, even for ER(+) breast cancer survivors.
Of course, if you increase the concentration of estrogen (or steroid cream) enough, you'll eventually get to a point where you do have systemic effects.
Thus, there are vaginally administered systemically acting estrogen products out there, such as FemRing.
Make sense?
@RoKhanna Elon came here on a student visa and illegally stayed despite not being g enrolled. He should have been ineligible to enter the US. Start there.