Looking for Method for Getting Audio File to Play in Vehicle

I am working on a project with Arduino MKR 1010 WIFI (esp32 onboard) and a microSD sheild (the Mem Sheild).

At a basic level, I am trying to download an audio file daily at a particular time so I can listen on my morning commute. I have no problem downloading the audio file, but all of the ways I have found to play the audio file are not ideal. I am looking for a method to store an audio file I download and play it in a vehicle with two available inputs - 3.5mm aux input or via USB port.

At a more in-depth level, I download a .MP3 file (<100 MB) to a microSD everyday at 5am and then listen to the file at 8am. After the download is complete, the Arduino goes into a standby mode until the next day at 5am. My preferred method to play the .MP3 would be through the USB port on my vehicle. If I transfer the .MP3 to a USB memory drive and plug it into the USB port on my vehicle, I am able to navigate the folders and play the .MP3. I would like to find a solution so I don’t have to transfer the file from the microSD to the USB drive. This is my preferred solution because the vehicle provides the user interface (play, pause, .MP3 file names, etc.) and the USB provides power so only 1 connection is required. Ideally, my final design would look like a USB memory drive and sit plugged into my vehicle’s USB port.

My backup method is to output on a 3.5mm jack and connect to the aux input. The issue here is, from what I have found, additional circuitry is needed to decode a .MP3.

Any advice on what direction to go with my project is appreciated. I have spent countless hours trying to figure out a way to have the Arduino act as a USB mass storage device without finding the knowledge I need. I am considering trying a no so elegant solution which involves physically disconnecting the microSD from the Arduino when it is in standby mode and attaching it to a microSD to USB drive adapter. I am posting because I may not even be aware of other solutions that may be better. I am looking to be pointed in the right direction. Then I can do the research.

At a more in-depth level, I download a .MP3 file (<100 MB) to a microSD everyday…

And… Why aren’t you downloading to a USB drive?

I would like to find a solution so I don’t have to transfer the file from the microSD to the USB drive.

…I have spent countless hours trying to figure out a way to have the Arduino act as a USB mass storage

Copying to a USB drive is the fastest and most straightforward solution.

You can add an MP3 shield with not much time investment but these things usually have some very-specific file-naming requirements and it could-end-up being finicky. And, you’d be using the analog connection.

I don't download to a USB drive because my issue will be the same - it will download to one piece of memory but cannot be played unless I remove it and plug it into something else. I'm hoping to leverage the fact that the memory needs to be read by the vehicle (never written to).

This would work as a solution if I could leave the USB drive plugged in and still write to it from the Arduino. If this is possible (and from my searches it is not because you cannot write to a USB using SPI protocol), it would be a good solution.

It isn't about time as much as convenience. I am willing to develop a new circuit to accomplish what I am looking for. Once I have a circuit that works for this project, I'd like to have a custom PCB made with everything I need and nothing more.