To unlock the bootloader of a Pixel 8a using ADB and Fastboot, follow these steps:

Ensure USB Debugging and OEM Unlocking are enabled on your Pixel 8a:
Open Settings > About Phone > Tap the Build Number 7 times to enable Developer Options.
Go to Settings > System > Developer Options > Enable USB Debugging and OEM Unlocking.
Connect your Pixel 8a to your computer via USB and open a terminal window in the directory where you have ADB and Fastboot installed.
Verify the device connection by running:
adb devices
If your device is recognized, proceed to the next step.
Reboot your Pixel 8a into Fastboot mode by running:
adb reboot bootloader
Your device will now be in Fastboot mode.
Unlock the bootloader by running:
fastboot flashing unlock
You will see a confirmation screen on your Pixel 8a. Use the Volume Up button to highlight "Yes" and press the Power button to confirm.
Once the bootloader is unlocked, your device will perform a factory reset and reboot into the system. You will see a warning message indicating that the bootloader is unlocked and that data stored on the device may be available to attackers.
Note that some users have reported issues with unlocking the bootloader on their Pixel 8a, even if the device is listed as unlocked on the warranty page. If you encounter such issues, it might be a server-side problem or a packaging logistics issue.

If you are unable to unlock the bootloader, you can try relocking it and attempting the process again. To relock the bootloader, follow the steps outlined in the Reddit post for relocking the bootloader on a Pixel 8/8 Pro.

Always ensure your device is backed up before attempting to unlock the bootloader, as this process will erase all data on the devic

Most of the best phones on the market run Android, and it's preferred by many for being a more open operating system than Apple's iOS. However, there are many things on Android that are also hidden from the average user. Thankfully, many of these capabilities can be accessed by using the Android Debug Bridge (ADB). If you're wondering how to set it up, we're here to help with that.

AYA Android ADB Desktop App
https://aya.liriliri.io/guide/
Download
https://github.com/liriliri/aya/releases/
AYA est donc une application de bureau open-source dispo pour Windows, macOS et Linux, qui regroupe tout ce que vous pouvez faire avec ADB dans une interface graphique intuitive. L’installation se veut ultra simple : téléchargez, installez, et c’est prêt. Y’a pas besoin de configurer des variables d’environnement ou d’apprendre des commandes cheloues… Il vous suffit juste d’activer le débogage USB sur votre appareil Android (dans les options développeur) et de le connecter à votre PC.

You normally need a PC to use the ADB tool for Android. But you can use it directly on your phone too. Here's how.