PRoot

PRoot is a user-space implementation of chroot, mount --bind, and binfmt_misc.

Setting up the environment
When creating a new rootfs, the first thing needed is a directory for the rootfs to reside in. For example, a rootfs could be created in :

If an installation has been previously created in a sub directory of the current root file system the above steps can be skipped.

Unpacking system files and the Portage tree (new installations)
When building a new install, the next step is to download the stage3 tarball and unpack it to the rootfs location. For more information on this process please see Downloading the stage tarball and Unpacking the stage tarball in the Gentoo Handbook.

External resources

 * https://wiki.termux.com/wiki/PRoot