If a country has joint ownership, does that mean that when two people get married all of their property is held jointly?

No. Very few countries have universal joint tenure, which means that all property brought into marriage or acquired in marriage in any manner is jointly held. A presumption of joint tenure for married couples means that there is a presumption that a married couple holds property acquired during marriage jointly. Often there are exceptions for property that is inherited or property that is gifted to one of the married couple. Thus, in most cases, even when married couples have joint tenure, ancestral land is excluded.

Some countries allow separate property to become joint property if the non-owner contributes to the value of the property (pays for improvements, for example).