As far as I know there is no configuration in this program.
While it isn't created by default, mpv supports a file called mpv.conf, which allows mpv behavior to be customized (globally, for particular media extensions, or by setting other custom "profiles" that refer to an arbitrary group of options and that can be used with --profile=). An overview of the general format of mpv.conf for the most recent versions of mpv can be found here ("Stable" version here).
How do I infinitely loop every video I play by defaut with MPV player?
Create a text file called mpv.conf.
Place the following line in that file:
ex. mpv.conf
loop-file=inf
Place mpv.conf under one of the following directories (depending on your platform):
# Linux
System-wide
/etc/mpv
/usr/local/etc/mpv
User specific
~/.config/mpv/mpv.conf
Windows
C:\users\USERNAME\AppData\Roaming\mpv\mpv.conf
Create "portable_config" in the same folder as mpv.exe
C:\path\to\mpv\portable_config\mpv.conf
If you want to infinitely loop a playlist, use loop-playlist=inf. You can add both options to mpv.conf to create an infinite playlist where each entry loops infinitely as well:
ex. mpv.conf
loop-playlist=inf
loop-file=inf
For usability, you can also add a profile to disable looping:
ex. mpv.conf
# global (default) options
loop-playlist=inf
loop-file=inf
"finite" playback profile (disable looping)
usage: mpv --profile=finite ...
Note that "finite" is an arbitrary name
[finite]
loop-playlist=no
loop-file=no
References (mpv)