I've tried to make it the most realistic as possible with my actual skills.
Modeled by me, textured with the help of Chocofur Free Shaders Bundle (https://store.chocofur.com/chocofur-shaders-bundle/blender_model/).
The scenario and lighting is Nishita Sky.
Rendered in Cycles, 256 samples with OptiX denoising ON. Blender 2.92
The flag itself uses Cloth Simulation with Wind Force Field.