Note: To edit #systemconfiguration files in #termux - add $PREFIX before /, then everything is the same.

E.g. to edit sshd configuration:
vi $PREFIX/etc/ssh/sshd_config

The PREFIX variable is already set in your termux environment. You can view using the echo command, and should see something like this:

~ $ echo $PREFIX
/data/data/com.termux/files/usr
~ $