Short answers, pulled from the story.
Dharmakirti lived in the middle of the seventh century between 600 and 670 CE. Chinese monk Yijing visited Nalanda between 675 and 685 CE and described Dharmakirti as a recent teacher from that era.
Dharmakirti defined a pramāņa as a reliable cognition in his text Pramāņavarttika 2.1ac. He argued reliability consists in compliance with an object's capacity to perform a function through perception or inference.
Dharmakirti defended the Yogacara theory of awareness-only against external realism by stating that no true objects exist outside the act of cognition itself. An object is necessarily experienced simultaneously with the cognition according to Pramāņavarttika 3.387.
Dharmakirti wrote the largest work known as the Pramāņavarttika-kārika along with other texts like the Nyāyabinduprakaraņa. Devendrabuddhi commented on his writings around 675 CE while Sakyabuddhi did so near 700 CE.
The Gelug school asserts he expressed Yogacara views while others claim Sautrantika perspectives based on his alternating philosophical approaches. Dan Arnold argues his alternating philosophical perspectives apply at different levels of analysis leading to modern disagreement.