By law. Primogeniture law. Younger sons did not inherit, only the firstborn. This also drove many younger sons of minor nobility and merchants to the colonies. Many indentured themselves willingly. They had a word for doing this but can't recall it.
Unless I’m mistaken, primogeniture applied to landed estates, not other forms of property.
Of course, in the earlier days we’re talking about, that would have been a distinction without much difference, since there wasn’t much in the way of other types of property.
Interestingly, when Anglicans dominated Ireland, they enforced equal division between his sons of the inheritance of a Catholic. Unless one of them converted, in which case he got it all.
Younger sons of nobles and gentry often had a commission purchased for them in the Army or Navy, or went into the Church.
In Catholic countries that generally meant their genes disappeared. In Protestant countries the clergy was often quite prolific.
Gentry and merchants often bought younger sons an apprenticeship in some trade, in which they could become journeymen and eventually masters. An apprenticeship, at least in one of the more desirable guilds, was expensive, and was viewed as the rough equivalent today of sending a child to college.