Who said anything about implementing? You didn't ask and I don't want to.

        I have some but wasn't my question.
        The problem is over years Android is more & more locked down and some variants even more so.
        While I was able to ssh/rsync to & from areas, the problem is in recent years Android even restricts access to your own SD card, which is where I wanted to rsync (Android PC being almost full but SD card being almost empty.)
        Elsewhere people describe how to ssh/rsync with termux, and I read termux wiki and you only get to copy to certain directory/folder on SD card, which defeats purpose.
        I'll mark this solved as what rkelsen said was true and I could ssh both ways (with connectbot or simplesshd or termux) though wasn't able to use solution to do what I wanted...
        I hope Android is replaced with more programmer-/system-administrator-friendly OSs that let you do more, like Ubuntu touch sort of did (though started snapd nonsense) but of course Unix & Slackware (and Gentoo, Devuan, non-systemd Arch forks, etc.) are better...
        The reason I asked is because MTP no longer works on the Android PC (for years) so I have to open case, remove SD card, put in desktop's memory card reader, or make archive/tarball to download, just to transfer files to Android PC...

Android isn't designed as a general purpose OS. It's highly customized for phones & tablets and low power devices, and seemingly only uses the Linux kernel as a matter of convenience. I wouldn't think it'd work well on a computer. It's quite heavily "chopped".

With android, the "system" partition is always mounted read only. You need to remount it rw if you want to change system files. As you mentioned your device is rooted you should be able to do this with standard commands.

For file transfers to & from Android devices, I use an app called primitiveftpd which is basically an ftp server. It works quite well, is fast & reliable over wifi.