One browser instance operates on one user data directory.
Scenario 1
$ ./<path/to/chromium/>chromium $ ./<path/to/chromium/>chromium
This first call opens up a new chromium instance in a new window (on the default user data dir).
The second call opens up another window for the same chromium instance.
–> This happens because they have the same user data directory.
Scenario 2
$ ./<path/to/chromium/>chromium $ ./<path/to/chromium/>chromium --user-data-dir="<path/to/non-default/user-data>"
This first call opens up a new chromium instance in a new window (on the default user data dir).
The second call opens up another chromium instance in another window.
–> This happens because they have a different user data directory.
Responsable code (v98.0.4711.1)