what is exactly Chalav Israel? if it is only the checking if there’s a spot of blood in the milk, how is this possible nowadays since the cows are milked through machines and it’s impossible to know if there was blood or not in her milk? besides, the milk today is pasteurized, filtered, heated, re-heated, homogenized... among so many other processes... the milk we drink today is completely different of the milk drank in the past. How's Chalav Israel is relevant in todays reality?

Chalav Yisrael has nothing to do with blood in the milk; the issue of a spot of blood was said regarding eggs. Chalav Yisrael is milk which was milked by a Jew or by a gentile under supervision of a Jew. Our sages forbade milk which was milked by a gentile without Jewish supervision to make sure that forbidden milk from an impure animal is not mixed into the kosher milk from a pure animal. (Talmud Avodah Zarah 35b; Shulchan Aruch Yoreh Deah 115, 1). The great sages of recent generations disagree on whether the rabbinic decree applies even in a case that there is absolutely no doubt that impure animal milk was not mixed into the milk. Some say that the decree only applies when there is an actual concern of milk from an impure animal; others say that the milk is banned in any case. According to Rabbi Moshe Feinstein ZT”L and other great Halachic authorities one can rely on the lenient opinion when it is clear that the milk is purely of kosher animals without any impure animal milk. Therefore in countries such as the USA that have government regulations strictly forbidding adding the milk of any other animal to cow’s milk, (and if one does so he will be punished and his business may even be closed down), one can use regular milk, especially where it is difficult to get Chalav Yisrael, and particularly for young children. However a Baal Nefesh ("spiritual person") should be stringent and drink only Chalav Yisrael. [ספר הכשרות להרב פוקס פרק ח סעיפים ט, יג, יד, ובמקורות, ובהרחבה באגרות משה יו"ד א סימנים מז-מט].