Hey,
@deicool.
I think every programmer has been at the stage where they want to reinvent the wheel with something: an OS, a web browser, a file manager, etc.
If you want to do this as a learning experience that is understandable - there's lots to learn from building an application whose niche has already been filled by several others. If, however, you intend to build this for fame, money, etc. My advice is:
don't bother. Because unless your application has a feature which isn't being fulfilled in other programs, nobody is going to use it, outside of a few tens or hundreds(maybe a couple thousand if you're lucky). So, take some time to think about what you're going to build and why.
Secondly(and I don't mean to be rude): get your head out of the clouds(this also seems to be a problem with many beginner programmers and again, no offense). Judging by your post on the C board, you do not seem like a confident programmer with extensive knowledge of any language(not from what I've seen so far anyway). If I were you, I would leave this just now and work on a few smaller projects(which there are plenty of lists for online if you're stuck for ideas). An application like this takes a lot of knowledge of file systems, command parsing, etc. and if you don't use any premade packages for commands, you'll end up remaking them all yourself - good learning experience, but again, you need to have the knowledge of how things work(especially for more complicated commands, like the UNIX
grep).
Since this is for Android too, you're advised to learn either Java or Kotlin; C or C++ are options, but I don't think the documentation for writing Android apps in them is as extensive as Java or Kotlin.
Alternatively(when you've learned a bit more and gained a bit more experience), you could begin examining the source code of Termux. Examine how the program is structured, how it approaches certain tasks, how is data structured, etc. You don't need to examine every single line of code in it - that would take you a lifetime and you'll be ripping your hair out because there will be so much more you don't understand than what you do understand. Instead, just take a piece of the program you're interested in(say, the application settings), look over the code and just play about with it(this is quite fun especially with open-source games, which I recommend you toy about with instead because it's much easier to see your changes).
Hope this helps!