Help needed some questions about madhab

Assalamoun alaykoum dear brothers and sisters,

To start off I dont have deep knowledge of islaam. So recently I met someone who told me that the true ahlul sunnah follow only one madhab in contract to salafis. Now I dont have a madhab. To be very honest when I look up daily practical matters of islaam I go islamqa or watch youtube videos of Assim Al Hakeem.

He told me this was not correct as the traditional ahlul sunnah learned islam (fiqh to be specific) from one madhab only. I asked what if a scholar finds a practise of another madhab closer to the sunnah and Quran. He told me thats impossible as the madhabs are founded on the basis of sunnah and quran. And to take some fiqh from one madhab and some fiqh from another madhab would be inconsistent and lead to following your desires.

Now I am not sure what the truth is.
He believes that Muslims, from Imam Malik up until Ibn Abd al-Wahhab, exclusively followed one madhab, but that after the emergence of Salafism, some Muslims started following the 'Quran and Sunnah'—in quotation marks, according to her.

Can you help with counterarguments?
Have there been significant scholars who acquired cross-madhab knowledge in fiqh?