Almost everything about how Passover is kept varies along lines that are older than they look.
Diaspora Jews outside Israel keep the first and last days as two-day festivals; Jews in Israel keep one. The reason is calendrical archaeology. When the month's start was declared by a court in Jerusalem and communicated by signal fires and then messengers, distant communities could not be sure which day the court had chosen and kept both. The calculated calendar removed the uncertainty entirely, and the doubled days were retained anyway, as the custom of the ancestors. Reform Judaism dropped the second day; Orthodox and Conservative practice retains it.
The seder itself, with its four questions, four cups and ordered plate, is rabbinic. Its structure has been compared to that of a Greco-Roman symposium, with reclining, wine in courses and formal discussion, and the comparison is standard in the scholarly literature. The Haggadah grew over centuries and continues to grow.
The Samaritans, a community of a few hundred people, still perform a Passover sacrifice on Mount Gerizim, actual lambs, actual altar, annually. They are the only community that does. Their calendar is calculated separately and their Passover sometimes falls a month away from the Jewish one.
Karaite Jews, rejecting rabbinic authority, revert to observation of the new moon and of the ripening barley, which also produces dates that diverge.
The plagues, the departure and the historicity of the exodus are argued elsewhere; the festival's structure is what is being described here.