I’m not sure what you don’t understand?
God created marriage and Christ confirms that marriage is for life. He declares that remarriage is not allowed. This law is for all people, not just for Catholics or Christians.
The Church distinguishes between divine law impediments to marriage-- for example, prior marriages, close kinship, etc-- and ecclesial law impediments-- things like who has to witness the marriage, etc.
Divine law impediments apply to all people as they are derived from God’s law. Ecclesial laws apply only to Catholics as they are adminstrative laws of the Church.
The impediment of a prior bond is a divine law impediment. Therefore, a person with a prior valid marriage cannot attempt another marriage.
So, a non-Catholic with a prior marriage would have present evidence that there was an impediment to a valid marraige if they want to enter a marriage with a Catholic. The tribunal would have to make a declaration of nullity. If they cannot do so, the person is bound to their first marriage and cannot marry.