Yurlok's middle ability is similar to the "mana burn" rule from years past. The loss of life happens at the same time the mana is lost. This loss of life doesn't use the stack and can't be responded to.

Multiple Yurloks on the battlefield won't cause additional life loss. For example, if two players each control a Yurlok and a player still has three unspent mana as a step or phase ends, that player will lose only 3 life.