I got a brand new phone and installed Termux a few weeks ago. Because of backwards compatibility of Android, you can install older apps. But Termux was not updated in ~2 years and no app targeting modern version can use permissions that Termux needs.

That is a problem on normal Android. But with Termux and certain other programs that have the right permissions, like the “File” app that came with my Samsung, you can access the entire OS and network drives. Also not what I ment, I see you haven't used Linux for longer, but it has a concept of mounting. You mount some virtual or real drive to a diretory and then this directory behave like a "portal". By this way you can for example mount online storage drive as it was a physical and for example install programs on it.

If you root your phone you can do anything. Not true. This is in theory, in practice you are very limited by Android messed up nature and lack of documentation from manufacturer side. Look up PostmarketOS, a real Linux distribution that community maintain. You can see how short is the list of supported phones, it needs to be ported for every device separately and takes huge amount of work. Look up PostmarketOS, it is a real mainline (means without much modifications) Linux-based OS. You can plug a monitor in and there would be no difference between PC and this, can install desktop apps.