To design a self-running app for your Raspberry Pi based on your requirements, here’s a step-by-step plan that you can follow. I'll guide you through the steps without giving the full solution, so you can piece it together.

### Steps to Implement

1. **Automating Raspberry Pi Startup Actions:**
   - **Systemd Service**: Use `systemd` to create a service that will automatically run when the Pi starts. This can manage your app’s startup logic.
   - **Startup Script**: Alternatively, you can create a simple shell script that runs on boot, either placed in `/etc/rc.local` or managed by `cron` with the `@reboot` directive.

2. **Detect USB Flash Drive on Boot:**
   - **Automounting USB**: Set up the Raspberry Pi to automount flash drives when plugged in. Look into editing `/etc/fstab` or using the `usbmount` package.
   - **Checking for MP3s**: Write Python logic that checks the mounted drives for files with `.mp3` extension. Use libraries like `os` or `pathlib` to scan directories.

3. **Copying MP3s to a Directory:**
   - You can use the `shutil` library in Python to handle file copying.
   - Ensure this logic only triggers when new files are found (so you don’t overwrite your existing directory unnecessarily).

4. **Running the Database Generator:**
   - Since you only want the database generator to run when needed, you can add a condition to check whether the MP3s were updated. For example, store a checksum or file modification time, and if there's a change, trigger the generator.

5. **Running the Flask Web Server:**
   - After copying the files and running the database generator (if needed), the script should launch your Flask web server.
   - Flask can be run as part of your startup service as well, once the other tasks are complete.

### Suggested Order of Operations
- On boot, run your startup script.
- Check for connected USB drives.
- If MP3s are found, copy them to the designated folder.
- Run the database generator only if new files are detected.
- Launch the Flask web server.

### Things to Consider:
- **Timing**: If there’s a delay for the USB drive to mount, you might want to add a short wait period or a retry loop when checking for the flash drive.
- **Combining Files**: You can combine the MP3-checking, copying, and database generation logic into a single Python script. Then, it can conditionally call the database generator only when new files are added, before starting the Flask server.