The coordinate system (actually the Spatial Reference System) is more than likely WGS84, because it's the default system for all GNSS systems.
This, however, is not all that is needed to georreference the images. You also need each image's corner coordinates, and here is where the problem lies. The lat/lon present in the images' EXIF are the drone's position when the image was taken. You'd first need to translate it to the image's central pixel's coordinate (given the drone's rotational parameters from its IMU and the height). Then you'd need to get the camera's calibration parameters to generate the corner coordinates. Do that for each image taken.
Quite the time-consuming and error-prone task, and that is considering you have access to necessary flight parameters. Easier is to ask the drone manufacturer directly and see if they have some native software for processing these images, or see if your drone is compatible with 3rd party software, such as Pix4D.
As an alternative, you may first mosaic all the images into one full-area image, then collect ground control points (using a handheld GPS would be fine, as your drone GNSS does not have better precision than that), and then manually georreference the whole image in QGIS or any other software.
You may follow this tutorial on how to do that. Just remember to grab photo-identifiable points, and to get a good amount of them, covering the entire span of the area (the closer to the edges of the image, the better).