How to remove the background from a photo on Android
A phone can cut a subject out of a photo on its own now, with no upload and no waiting. What decides whether you end up with something usable is the format you save it in, and that is the step most instructions leave out.
Transparent means PNG, not JPEG
A JPEG has no way to record which pixels are see through. Save a cutout as a JPEG and every transparent pixel gets filled in, usually with white or black, and the edge you just cut is a rectangle again. Transparency needs a format with an alpha channel, which in practice means PNG or WebP.
This is the most common reason a cutout "does not work". The cutout was fine. The export threw it away.
Option 1: what your phone may already do
Recent versions of Google Photos and Samsung Gallery can lift a subject out of a picture on their own: press and hold on the subject, wait for it to be outlined, then choose Copy or Share. It costs nothing to try and it is already installed.
The catch is what you are handed at the end. The feature is aimed at stickers and at pasting into a chat, so depending on the phone and the version you may get an image with a background already flattened behind it, and no say over the format or the size. Try it first, then check the result using the test at the bottom of this page.
Option 2: a website that does it
There are many, and they work. What they have in common is that your photo is uploaded to a server you do not control, processed there, and sent back. For a picture of a product that is usually fine. For a photo of a person, a document, or anything you would not post publicly, it is worth knowing that is the trade you are making, because the page rarely says so plainly.
Option 3: an app that does it on the device
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Open the photo in the app.
Either pick it through Android's own photo picker, or send it to the app from another app's share sheet. The picker hands over only the image you chose, which is why an app that works this way does not need permission to see your gallery.
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Let it find the subject.
The detection runs on the phone. It is quick, and it works without a connection once the model it uses is on the device.
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Choose PNG before you save.
This is the step that matters. If there is a format setting, PNG or WebP keeps the transparency and JPEG destroys it.
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Save, and find it in your gallery.
The cutout is written as a new file. Your original photo is not modified.
How to check the transparency actually survived
A transparent PNG and a white background look identical against a white page, which is how a bad export gets as far as being sent to somebody. To tell them apart, put the image somewhere dark: drop it into a chat with a dark theme turned on, or set it briefly as the picture in an app with a black background. Real transparency disappears into the dark. A flattened export shows up as a white block.