Final answer:
The Filipino architect's first project, known for its use of concrete and simple design, and collaboration with National Artists, was the Church of the Holy Sacrifice.
Step-by-step explanation:
The first project of the notable Filipino architect that utilized concrete, floating volume, and simplistic design was the Church of the Holy Sacrifice. This project was not only groundbreaking in its architectural style for the Philippines, showcasing thin shell concrete domes, but also involved collaboration among several National Artists. The Church of the Holy Sacrifice is now celebrated as a National Historical Landmark and a Cultural Treasure, as recognized by the National Historical Institute and the National Museum respectively.