How did guilds form in medieval society?

A guild is an association of artisans and merchants who oversee the practice of their craft/trade in a particular area. The earliest types of guild formed as organizations of tradesmen, belonging to: a professional association, a trade union, a cartel, and/or a secret society. They sometimes depended on grants of letters patent from a monarch or other ruler to enforce the flow of trade to their self-employed members, and to retain ownership of tools and the supply of materials, but were mostly regulated by the city government. A lasting legacy of traditional guilds are the guildhalls constructed and used as guild meeting-places. Guild members found guilty of cheating on the public would be fined or banned from the guild.

Explanation:

The medieval guild was established by charters or letters patent or similar authority by the city or the ruler and normally held a monopoly on trade in its craft within the city in which it operated

interdependence is when all participating groups and individuals depend on each other. if you depend on one anothers goods, you end up trading in order to get the items you're looking for. (for ex: if i have food and you have water, we trade in order to depend on each other to live)