hmm.. what do you mean app? I'd not heard of those. I just assumed people people made their thumbnails on a computer on editing software.. Are peoples getting by doing this on their mobys. iPads?? I use photoshop any way. You can do quite a lot with that.
I haven't heard of "PIXEL LAB", although I have used Canva in the past for non-YouTube related projects. From what I've seen with regards to YouTube thumbnails, Canva is mediocre at best as it's mostly made up of pre-made templates.
Instead, I use possibly the closest thing to Photoshop CC/CS6 that is web-based - Photopea. It's a nice online version of Photoshop that gets the job done. I've actually switched from the actual Photoshop application to using Photopea. Might make a video on it in the near future.
I've also used Pixlr in the past, which is okay, but again, not as great at Photopea's editor.