Android on Raspberry Pi
The Android operating system will run on the Raspberry Pi. Take note that it is not 100% stable and is unoptimized for the hardware, meaning the performance on the Pi will be subpar compared to other OSes designed for it. Android will install and be mostly usable on the Raspberry Pi 3 and newer models. Android cannot be installed on the Raspberry Pi Zero since it uses the ARMv6 architecture, which has been unsupported in Android for years. If you don't have a touchscreen display, Android supports a keyboard and mouse and most apps will work fine with it.
You'll need an Android image and you will need disk image writing software to write the Android image to the flash drive.
Popular Android images/versions for the Pi are emteria.OS and LineageOS.
A very popular disk image tool is Etcher, which is what most people use. It does not need to be installed and runs on Windows, Mac, and Linux.
emteria.OS is probably the most popular, however, it is not free and costs money. The evaluation version is free but lacks many features. The personal version costs €19 (around 20 USD) whereas the business version requires a quote from the sales team. Emteria is more suited for industrial use but it does contain a full build of Android and has a good support team.
LineageOS is an alternative to emteria which is free. It contains builds for Android 7, Android 8, and Android 9. It supports Google Play and other Google apps.