It is possible to read a QR code on Android phone without downloading or installing any apps. This option should be available on mobile phones with a newer Android versions, so it may not work on every device.
Android phones should have pre-installed Google Assistant
or Google Lens
app. These applications are able to read QR codes. Google Lens is a tool that allows to search what you see, by using an image recognition. It uses phone's camera to recognize and read a QR code. To use these apps for image recognition purpose, you will probably need an internet connection.
If you are using a Google Assistant, you have to switch from the Voice search (microphone icon) to Google Lens image recognition tool (camera icon). Alternatively, depending on your device, you can run Google Lens by tapping on the camera icon
in Google search bar: