Where is the folder that Termux defaults to? The Termux app is a terminal emulator. If I create a file or a folder there, and then use another file explorer app to browse my device, where can I find these files?
You don't need to root your device. All you need to do is run the following command on a non-rooted (or rooted) device to allow termux to access your existing directories, particularly /storage/emulated/0:

termux-setup-storage


this creates a new directory in termux, ~/storage, which contains simlinks to /storage/emulated/0 and can be accessed by a standard gui file manager.

You will need to grant file access to termux when prompted after you run the command.

See here.