How mount (or auto mount) an usb stick formated with ext4 and be visible or working with apps on Android 5 , Marshmallow and later versions The question appears in variate forms [1] just to say it is ridiculous on Android that ext4 is the default file system doesn't recognize usb pen formatted with ext4 (but android 4.4 reconize it without any problem), as a Linux user this is a big regression , and I still not found a way to usb be detect as usb , like pens with FAT32 or NTFS. I have root on my device . 

[1]
Mounting a ext4 formatted sdcard on Android Marshmallow 

Partition gets unmounted automatically in Android Oreo 

External Ext4 card mounted only for root (!) on Android 5.1 64 bits

How to use an ext4 formatted sd card as external storage with working app permissions?
We still need root. First I need make a dir in /data/media/0/ (/sdcard) 

mkdir /data/media/0/usb1/


which means usb is not detect as usb ever but I can read data from apps that it is what I want .

After I made a script with :

su -mm -c "mount -t ext4 /dev/block/sda1 /data/media/0/usb1/"; 
su -mm -c "mount -t ext4 /dev/block/sda1 /storage/emulated/0/usb1/"


and after with https://play.google.com/store/apps/details?id=os.tools.scriptmanager

I made a favorite script and with the add-on to create a launchers, I made a icon in home screen (shortcut I preferred instead widget) and on click it, it mount my ext4 USB pens.