How does medieval inheritance worked for non eldest children regarding liquid assets such as money, plate, or other riches? or did the eldest male offspring got the land and money?
As anyone who’s played Crusader Kings II or III can tell you, it depends entirely on what the laws and customs of inheritance were in any given time and place in the Middle Ages.
Different cultures, even if they were geographically proximate, could have very different laws - the Welsh custom of cyfran (gavelkind that included illegitimate children) held sway the Welsh side of the border, and then a few miles away on the English side patrilineal primogeniture would be the law. (Naturally, this led to a lot of legal disputes in mixed families.) Customs could also differ within a given polity or culture: while patrineal primogeniture was dominant in most of medieval England, the custom of “Borough-English” (patrineal ultimo-geniture) was widespread in southeast England.
And as anyone who’s read about the Hundred Years War knows, inheritance laws could change according to political need: the application of Salic Law to the inheritance of the throne of France was more or less invented post-hoc to justify Philip V inheriting ahead of Princess Joan, and then again (when Phillip V and Charles IV both died without sons) to justify Philip VI inheriting ahead of Edward III of England. Similarly, the inheritance of the House of Wessex shifted back and forth between the presumption that the King’s son should inherit to the King’s brother should inherit, depending on the political necessity of the time.
To answer your question about liquid assets...primogeniture tended to apply more strictly to titles (which can only be held by one person at a time) and lands (where concerns about economies of scale vs. subdivision were dominant) than to moveable property, and it would be rather rare that the younger children would be left with nothing. (After all, that’s not very condusive to getting them to accept the outcome without a fight.) What you tended to see is that the oldest son would get the biggest share of liquid assets, but that there would be some bequeath to the younger sons.
However, liquid capital had significant limitations in an era without free markets in land, labor, and money - it was rather difficult to turn that lump sum into a continuing income if you couldn’t easily buy land. This is why younger sons often went into the military (where you could make a fortune on loot or possibly be given/take conquered estates), the clergy (where you got room and board at a minimum, and potentially could get quite rich from benefices), or into trade (where you could invest your inheritance in income-producing trade).
