Good luck with this: the front end is a React client that is quite complex.
Maybe if you could explain what you are trying to do we could give you some hints
Thanks for the reply. For example, I want to make UI RTL or install another system design like Ant Design, and add Arabic language to it.
I see too many files and assets in here, and it is too complex for me
You could certainly design your own UI and call (almost) everything via the Cells REST API — but this would be a tremendous amount of work (essentially replicating all the functionality of the React client, but using your own preferred framework).
The listed Cells API is for the currently stable version, 4 (version 5 is under development and may have a slightly different API…).
But you can certainly start with some basic things, calling the main functionality you need for your purposes, and slowly work your way adding further features… it’s a long API