A guess: All used parts of the original image are clipped with super carefully drawn selections to separate layers. Then some of them like the stars of the flag are duplicated as needed. The fighting riders in the bottom are used as is, only partially covered. Many used original items are scaled to a new size.
Much items like the sky and the mountains are redrawn, only the colors are copied. The clothes and the hands of the woman are partially redrawn to cover the missing areas.
The new face can be a drawing or it can be a filtering result of a photo. I guess it's a drawing, cartoonizing filters tend to make unwanted rubbish with do not fit into the wanted style.
We do not know is the face copied from another image or made for this composition.
Summary: This is a new composition which uses some old shapes and colors and contains also some new parts. It's made by a skilled person. It's not especially difficult if the face is copied but needs well sharpened skills to detailed work with multilayer images and several hours of time.
What's copied can be found by carefully comparing the images side by side and also layered with blending mode difference and then scaling and moving the top layer so that the suspected copy is in the same place in the same size. Nothing will match pixel perfectly because the images are differently scaled and many copied parts are scaled to different size. You'll find the edges of the colored areas have 1 px differences everywhere.
Tools: Photoshop and a high resolution original, a screenshot is useless. If only the shown 600x600 image is available redrawing in higher resolution or as vector in Illustrator can be a solution, but the needed amount of work grows easily to 1000%.
Before redrawing one should try an Image Enlargener such as On1 Resize or Smilla (freeware) + others. It can guess and regenerate succesfully sharp borders and thin lines. Clipping the wanted pieces apart becomes possible. Here's a snippet, enlargening is to 600%
It's a bit sharper than the original 600 x 600 image in the same size: