If someone enters the US illegally is it impossible to get residency if they marry a us citizen?

I don't know where are you from but when I read US immigration laws in my country of origin, if the immigration discovers that you entered in the US illegally, that person will be deported and will never land in US soil ever. Will the American partner follow the other?

If that person will marry a US citizen, that doesn't make him automatically eligible for US citizenship because that has to be applied in the US embassy starts with questioning and investigation. You will be like...

"How did you meet?" I was in the US..
"How did you go there?" Visitors visa. I visited my aunt..
"How long?" 4 yrs..
"Visited with aunt for 4 yrs? she supports you with everything?"
Yeah..
"But you got no job?" uh ahhhh...

Immigration officers are well trained and can quickly smell something fishy. Trust me, you.. Too risky dear... don't think about it! If I were the illegal immigrant, go back to the homeland, start everything clean while no reports yet for deportation and process things right.
"
I knew Serbian who faithfully married but got divorced from his American wife before getting his green card. He got SSN, can work and everything it's just he can't visit Serbia because he can never return in the US. Not sure if that makes him illegal alien but surely he didn't enter the US illegally. Hopefully someone can answer me about that.

Hmmm thank you for the response.

If you married illegal alien who entered US without inspection, your spouse cannot adjust his/her status in US, he/she must return to his/her home country while I-130 being processed. Most important thing, you will need an experienced immigration attorney.

If you married illegal alien who entered US without inspection, your spouse cannot adjust his/her status in US, he/she must return to his/her home country while I-130 being processed. Most important thing, you will need an experienced immigration attorney.
Hmmm then it can be started here in the US but he would have to go back in the process?

It depends on the exact facts of the "illegal" entry.

Generally if there is an EWI entry, then the applicant will need to be grandfathered for an old law called 245(I), but that ended April 30, 2001. If so, then the I-130 is filed here and you apply for the IV with a waiver at your home consulate.

If false documentation was provided, and there is an exceptional hardship to the USC, then you may qualify for a waiver for the misrepresentation and you may be able to AOS here in the US.

If a false claim to US citizenship was made, permanent bar, no waiver, ever.

You should consult with an experienced immigration attorney. Very good law firms in CA, NY, FL and TX.