As you can see in the picture, this Cambridge dictionary app has some dictionary tabs, such as: Cambridge, Oxford. Those dictionary tabs have a link of their website. How can I add more dictionary tabs like collins, merriam dictionary?
I downloaded the APK file and loaded it into Jadx.
The dictionary tabs and their target are hard coded in the APK DEX byte code (it is a large switch case code entry in the MainActivity that sets the target url of each of the 6 tabs). Therefore modification requires decompiling and recompiling skills - however such actions are out of scope of this site.
Therefore the only answer we can give to you is to contact the author of the app and ask if other dictionary sites can be added.