It's a huge combination of factors - raw skill is one obvious one, but there's also things like the player being able to comfortably move countries without being homesick/unsettled, language, the league and team they came from, quality of opposition and their respective strengths and so on.
Top level leagues tend to be judged on coefficients, though there's some debate as to whether they give a true statement about quality given that some bigger clubs come from relatively ****e leagues and some strong national teams are fed by mediocre (by PL standards) talent at league level. Second tier clubs are generally judged next to the top flight of the respective nation, but it isn't as cut and dry as that, so that's when you'd need to be much more detailed. So the Championship would comfortably be the strongest second tier, followed by Serie B and, Segunda Division and Ligue 2. But some nations, like Scotland and Norway, tend to have very poor standards of second tier football, so talent from there isn't in abundance and if you're a Championship club they're not leagues you'd probably bother looking at a lot of the time. Which is a shame because that doesn't mean the league is a write off, but English clubs tend to wait until a player moves to a bigger club and proves themselves before making an offer because they have the finances to do that. I imagine Southampton were watching Ross Stewart for a while before he moved to Sunderland from Ross County, but were happy to let him have a season at Sunderland to see if he developed. That one backfired a bit
If a player was moving from Kosovo to Serie B, for example, that's a big step up, so the player would have to be very good. It wouldn't matter if the lad had scored 30 goals in the Kosovan top flight, that might pique the interest but it won't guarantee him the move if he's only going to end up being bullied and sulk. In fact, we took Malta international winger Clayton Failla on trial in our bounce back promotion season after he scored 45 goals in 92 appearances appearances for Hibernians in Malta. He didn't look anything like good enough when he trotted out against Hannover, but he went back to his home country and had a decent enough career there. We also had players like Martin Pusic, Tijani Belaid and Norbert Balogh come through that on paper would have looked like decent signings, but ended up having little to no impact at all because they just couldn't break through.