Remember that ducta, on its own, is a participle. It comes from ducere, but it acts as an adjective "led", not as a verb "lead".
In Latin, there are no finite verb forms for the perfect tense passive. The language just doesn't have them. So to indicate this meaning, Latin-speakers used periphrastic or compound forms: they had a perfect passive participle, so they combined that with forms of sum to get a finite perfect passive meaning. "Led" as an adjective is ducta, and so "she was led" as a verb is ducta est.
English, in fact, does the same thing for all of its passives. English doesn't have any finite verb forms for the passive voice, but does have a passive participle, so we just combine that participle with forms of "be" to get passive finite meaning: "is led", "was led", "will be led". The difference is that English has to do this for all its passives, while Latin only needs it in the perfect (and pluperfect and future perfect).
At its core, though, ducta still acts like an adjective. That's why it marks gender and case, which finite verbs don't: she ducta est but he ductus est and it ductum est. Very literally, "she is (est) in a state of having been led (ducta)".
…and, just because this wasn't confusing enough, Latin actually adds one more complication on here. In Latin in general, forms of sum can generally be dropped if they're clear from the context. Which means you could actually form this sentence without the est! But it's important to remember that the est should be there, even if it's just implied and not spoken: the sentence needs a finite verb, and ducta is a participle, not a finite form.