How to research competitor thumbnails for your own channel
Looking at what works in your niche is a good way to improve your own thumbnails, as long as you learn from them rather than copy them.
Collect 10 to 20 thumbnails from videos in your topic, saved in the largest size, and place them side by side. Note the patterns: how many words of text, which colors dominate, whether there is a face and how close the subject is to the camera.
Then ask what is missing. If everyone uses blue backgrounds and small text, a bold warm color and fewer words may stand out. Design your own image from these observations and check it at phone size before you upload.