Hi irdr
Yes this is a common requirement (to a phone or for the car)
First of all getting it off your B2
1) You could export the contents of your B2 to a USB Thumb drive/stick or HDD and then load this into your computer (the USB device would then also be a backup for your B2)
2) You could turn on the NAS capability of the B2 ( > Settings > Maintenance >
Start NAS ) from the front panel controls
which allows you to see the content of your B2 from your PC/Mac.
3) copy the content of the B2 to a local area on your PC/Mac.
Second converting to Mp3
You are correct while FLAC the best compression format because it is lossless, most phone and cars want the music further compress to the lossy Mp3 format.
that people here have used to convert to Mp3.
However please also note that you will run into difficulties with music "tags" that phones and cars expect to have embedded in the music files. The Mp3 files will play OK of cause but the "Apps" use/need the tags to display the content of what you have uploaded.
To me "Tags" are still a bit of a
black art, and I have only just begun to explore them. However people here have used a free pen source utility called "
mp3tag"
https://www.mp3tag.de/en/to add these.
hope this helps
Fred