Hello i try transfert mp3 to my phone and in my Google Pixel 3A XL i not have MTP any solution ?
The default is Charging Only, tap the silent notification to choose File Transfer.
on manjaro or on my phone ?
Your phone.
Don’t find it on my phone you can explain more where is please
If you don’t see the notification, your phone isn’t being detected. Try another USB port or cable.
On your phone, tap the “Charging this device via USB” notification.
Under “Use USB for,” select File Transfer.
i have file transfert/android car , share connection with usb , midi and PTP not have MTP on my phone
File transfer is MTP (Media Transfer Protocol). 
ok now i got file or folder udi=/org/kde/solid/udev/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-2 dont exist
revice this error in dolphin name KIO Client