Using amiibo .bin files with JoyCon Droid. Not all phones can use amiibo. Check below for either the Samsung or custom ROM requirements.
You won't get help for finding and downloading amiibos. You must use your own amiibo .bin backups. The FAQ explains this.
Samsung vs Custom ROMs (more phones)
Samsung is the only known smartphone manufacturer that usually has a large enough Bluetooth HID_DEV_MTU_SIZE found in the stock system firmware. Most of the time you do not need to flash a custom ROM if you have a Samsung device. A custom ROM on Samsung devices might only be necessary if you have an older device and need to upgrade to Android 9 (P) or greater.
With custom ROMs, you can use amiibos on almost all other kinds of Android devices. The ROM you install needs to have HID_DEV_MTU_SIZE 512
Most ROMs that are LineageOS 16.0 based and higher built after 2020-07-September are highly likely to have this and be capable of using amiibo. It's not yet possible to check this on your device besides looking at your ROM source. For example. [The Patch introduced on 7th of September 2020 is only for the AOSP Bluetooth Stack. If an Android device uses the Qualcomm Bluetooth Stack, HID_DEV_MTU_SIZE is still set to 64.]
Custom ROMs that have HID_DEV_MTU_SIZE 512
LineageOS 16.0 and higher (built after 2020-07 September)
Many more if based on LineageOS and built recently after 2020 07 September
You can find your ROM's Build date by checking the date on the ROM you flashed or going to your phone's Settings app → About phone (open System first on some phones) → Android version (open Status first on some phones)→ Build date
Please be aware that the app can be unstable and it can take you multiple tries to get connected properly and "scan" amiibos.
How to use amiibo:
You first need to enable NFC in JoyCon Droid Settings before you can use amiibo
Tap the menu icon in JoyCon Droid
Turn on Enable NFC
You will get a warning if you're using a non-Samsung device or custom ROM and also letting you know that this is experimental and might not work. Tap Continue if you believe to have a compatible Samsung or a phone with a custom ROM that has Bluetooth HID_DEV_MTU_SIZE 512.
Now tap Select to find an amiibo .bin file from your device. Put all of your amiibos in a folder so you can find them easily.
Find and select an amiibo .bin that you want to use.
You will see what amiibo you currently have Selected.
The .bin you chose can be seen under NFC Binary Path. You can now try to connect JoyCon Droid to your Switch and use it in your game.
If you want to select a different amiibo, you must change it just like above in NFC Binary Path.
You may need to also lower the Packet Rate to 10 pps
Make sure you chose a .bin file. You could have a corrupted .bin such as from an incomplete download so try redownloading it.
Try out different amiibos.
Make sure you're using a Samsung phone OR a custom ROM that was built after 2020-07 September and has Bluetooth HID_DEV_MTU_SIZE 512 [The Patch introduced on 7th of September 2020 is only for the AOSP Bluetooth Stack. If an Android device uses the Qualcomm Bluetooth Stack, HID_DEV_MTU_SIZE is still set to 64. Qualcomm however forked the AOSP Bluetooth Stack.]
Not all amiibos can work.
amiibos that write data cannot be used. JoyCon Droid does not yet support writing data to amiibos.
Examples of amiibos that can't be used are Wolf Link in Breath of the Wild, Splatoon 2, and many other amiibos that need to be registered can't be used.