You can customize the icon shown on the navigation tabs in your app by uploading SVGs.
Tab icons should be square SVGs. Advanced SVG features like embedded text, raster images, and gradients are not supported. The app sets the color of each icon based on the context in which it is shown, so any colors in the SVG images that you upload will be ignored.
There are a number of sites where you can find free icons for commercial use without attribution, such as https://uxwing.com or https://www.svgrepo.com. There are also some sites like https://thenounproject.com/ that offer royalty-free licenses to SVG icons for a nominal fee.