Large masses warp space-time and that shows up as gravity, in the absence of a graviton particle how do objects interact with the curved space-time? How do they know(wrong word I know) that space-time is curved?
In short, they don’t. Spacetime curvature is a mathematical model for our puny primate brains. It’s not what’s actually happening in nature. In fact, spacetime curvature, which was introduced by Minkowski, was initially called “superfluous erudition” by Einstein, but he later agreed that the model jived with his equations.
Gravity is a force field, just like a magnetic field. A gravitational field acts on masses moving through it, just like a magnetic field acts on magnetized objects moving through it. Maxwell developed the equations describing electromagnetic fields; Einstein developed the equations describing gravitational fields.
Fields mediate energy through waves. Waves can be quantized as particles. Electromagnetic fields mediate energy through light waves. The quantized particle of a light wave is a photon. Gravitational fields mediate energy through gravitational waves, which we have detected in recent years (look up LIGO). In theory, the quantized particle of a gravitational wave is a graviton. But unlike photons, we cannot detect individual gravitons and may never be able to. We currently can only detect some of the largest gravitational waves in the universe, which would be compromised of ginormous numbers of gravitons. We have no idea how to detect gravitational waves small enough to discern the existence of individual gravitons. Therefore, although they must exist, gravitons remain undetected and theoretical.
Secondly, why are photons which have no mass affected by gravity?
Covered above, but to reiterate, photons do have mass. For a practical example relevant to this thread, we have flown solar sails in space, which work by gajillions of photons imparting inertia to the sail when they collide with it (just as gajillions of air molecules impart inertia to a cloth sail when they collide). If photons were massless, they could not impart inertia to the sail.
Hope this helps.