This isn’t trolling, and I’d be glad if the Rabbi would explain.

What I was thinking is that there is a problem with the steam and with heating the iron itself (a glowing piece of metal). In addition, there is a rabbinic prohibition in the ironing itself. And finally there is the issue of electricity.

On a Jewish holiday there is no problem of steam, or cooking / burning a glowing piece of metal. And during twilight there is no issue of electricity. So the remaining question is whether there is a problem with the ironing itself.

What I wrote, that ironing is rabbinically prohibited, is only based on what I understood, but could there be a prohibition here of whitening / laundering (a Torah-level prohibited labor)?

So apparently this is unintentional trolling. What does twilight have to do with this? Because of the rule of rabbinic prohibitions during twilight? That’s a strange way to ask a question, if all you mean to ask is whether this involves a Torah-level prohibited labor or not.
The heating of the iron itself could be Torah-prohibited (although that is open to discussion). The ironing itself is not whitening / laundering, even though in the language of the Talmud, in the laws of the intermediate days of a festival and mourning, there is some conceptual mixing between our ironing/laundering and their laundering/pressing. Why would our ironing be whitening / laundering? What exactly are you whitening there? You’re only straightening the fabric. Only if the ironing takes part in the cleaning itself is there room to discuss all this. And you’re assuming that heating on a Jewish holiday is permitted even when not for a need (food preparation), and that too is not so simple.

There’s no trolling here.
If the iron was heated before sunset, then seemingly one could continue and also turn it off before nightfall (since there is no rabbinic prohibition during twilight)?

Okay, in any case, thank you for the answer.

And another question:
If the soup ladle is inside the dishwasher on a Jewish holiday, is it permitted to open it to take it out and close the dishwasher so that it will finish the program?
(Under the assumption that you are not changing anything electrically.)

If we’re looking for trolling, then I’ve now found a long discussion in the responsa Minchat Yitzchak (7:33) about whether it is permitted to prepare legumes from a Jewish holiday that falls on Friday for the Sabbath.
What’s the answer? Of course it’s prohibited.
1. It seemed that it should be prohibited because the principle of “since” does not apply. In the end he rejects this reason, since Sephardim are common among us.
2. It is no better than someone fasting a dream fast, who is prohibited from cooking for others. And anyone who cannot eat it that day cannot cook it.
3. Perhaps he may come to eat it on the holiday.

As for the ladle, why not?

Thank you very much.
Ironing with cornstarch counts as the prohibited labor of whitening / laundering, right?

I don’t know what that is. If it’s a whitening / laundering process (that is, a process that cleans and not just smooths), then apparently yes.

A process that whitens sheets and gives them a sheen.

Now that I think about it, if the iron is hot before sunset and has no water in it, there is no obstacle to ironing even during twilight on Friday evening and then turning it off before nightfall.

As I wrote above, in my opinion you are conducting the discussion incorrectly from a methodological standpoint. If you want to clarify the question of what is rabbinically prohibited and what is Torah-prohibited, it is better to clarify that directly and not through questions about twilight, which is only a practical ramification and depends on additional factors.
In particular, you are assuming various imprecise assumptions here regarding rabbinic prohibitions during twilight. According to most opinions there is no blanket permission for every rabbinic prohibition, and there are many disputes about this. For now, see a bit here:
https://www.yeshiva.org.il/ask/80861

That’s what actually happened.

If possible, I’d also appreciate an expansion on the matter of “since it was permitted for a need, it was also permitted when not for a need,” so as to understand whether heating the iron itself is in fact Torah-permitted on a Jewish holiday or not (the Rabbi left the point somewhat unclear).

Thank you very much for the reference regarding rabbinic prohibitions. It gave me food for thought (some of which I understood intuitively and some less so).

On “since,” see here:
http://etzion.org.il/he/%D7%96-%D7%9E%D7%AA%D7%95%D7%9A-%D7%A9%D7%94%D7%95%D7%AA%D7%A8%D7%94-%D7%9C%D7%A6%D7%95%D7%A8%D7%9A-%D7%94%D7%95%D7%AA%D7%A8%D7%94-%D7%A0%D7%9E%D7%99-%D7%A9%D7%9C%D7%90-%D7%9C%D7%A6%D7%95%D7%A8%D7%9A

Thank you very much.

I apologize for the accusations, but I really thought this was trolling. It wasn’t presented as a practical halakhic question but as a conceptual discussion.