Verse 51 of Surat al-Ma'ida refers to not taking Jews and Christians as rulers. So how can Muslims establish friendship with the People of the Book?

The word "friends" appears twice in this verse. One represents the Arabic word "awliya," meaning "protectors, those responsible in the sight of the law, parents, owners, or rulers." The other is the word "yatawallahum." This also means "to assume the care of, take command of, or to assume the administration of." The word "friend" in this verse therefore bears the meaning of "rule." Allah tells Muslims not to enter under the rule of Jews and Christians. In other words, the meaning of this verse is not, as some people say, that we should not enjoy good relations with Jews or Christians. It is perfectly natural for a Muslim society not to have a Jewish or Christian leader. Since both sides will act in the light of their own beliefs, a Jew by his beliefs and a Christian by his, and a Muslim will act in the light of his beliefs, situations that conflict with Muslim beliefs will inevitably arise, so this provision is totally normal. But apart from that, Muslims have very close relations with the People of the Book. A Muslim can marry a woman from the People of the Book. For example, our Prophet (saas) himself married a woman who is a member of the People of the Book. One can go to their weddings and funerals, eat their food, do business with them and live side by side with them as friends and brothers. This can clearly be seen in the practices of our Prophet (saas).