1. Can you share what tool you are using to create thumbnails, please?
Sure! I create my thumbnails using Affinity Photo, so they are all custom, but I also often use tools like Canva to make some of the graphics that I add to my thumbnails. I have gotten positive feedback so far even though I am not a big YouTuber. Of course, I am a professional photo editor, so that helps. I really do suggest you learn how to use a photo editing program like that one or Photoshop, though. It is very much worth the time even if you can just learn how to cut out backgrounds or add drop shadows. There are so many tutorials out there to follow. It is okay if you do not understand it to begin with.
2. Can you give some tips & tricks for thumbnails, please?
With thumbnails, I suggest that you look at the most popular creators in your niche and then take inspiration from them. Generally speaking, high-contrast, vivid, and visually interesting/layered thumbnails go a long way in attracting people, but there are cases where simple ones go even further. I suggest that you (if you have access to the feature... you may have to have a certain amoount of subscribers first) create multiple thumbnails and then let YouTube do their A/B testing to see which one gets your the most engagement. I have learned a lot about what works for my channel in particular that way.