Where does the party talents comes from?From the players?They acquire them and they decide to give it to the party?

Yes, an individual player sockets a talent to the party sheet.

It is then useable by all. Constant/not recharge ones work best. Helps keep talents you're not socketing yourself handy.

Roleplaying wise, good if people narrate how the player who socketed it influences/advises etc. so they get the benefit.

Thank you for the wise and clear answer!

Yep it is always a Huge source of roleplaying between players. Nobody should be able to share a talent without à good catchy line or Ingame reason