Unless somebody says explicitly "no women", there is no discrimination in my opinion.
Because interaction goes both ways. A big influencer on women not being interested could be a societal expectation that is not a job for them, which you’re unknowingly reinforcing.
This is particularly important when it’s not “mechanic jobs” but “senior jobs” for example. Only male workers being “proposed” leadership positions over time leads to a statistically significant imbalance.
At some point, we have to face the fact that there are two kinds of freedom: The freedom TO something-or-other, and the freedom FROM something-or-other. And the two are often in tension, requiring actual judgment calls and weighing of values, because there is no one perfectly crafted set of objective rules to sort that mess out.
Some people care about the freedom from algorithms not showing them ads for jobs they are qualified to do and pay better, but the companies would prefer the freedom TO primarily hire whomever they please and advertise to whomever they please. Those two freedoms are in tension.
If the freedom from gender discrimination in the marketplace freedom doesn't matter to you, or matters les than the freedom for someone else TO advertise only to men, well, I can see that you are consistent in your beliefs of things I deeply disagree with.
You have however written a thing here that’s fine - it’s totally fine if your advert is seen more by men. But what you want, and what we as a society generally want, is for those ads to be shown to likely candidates regardless of gender. Given two equally qualified people, do you want your ads to only be shown to one of them, because the other is a woman? I assume not because you want to hire th best person not the best man.
The issue isn’t that the ads are shown to more men because they target things like “has said they have worked as a mechanic and are looking for a job” and that happens to be more for men, the accusations is that Facebook is specifically using your gender to determine what job adverts to show you.