Where is the autoload.php?

[codemasher]

Where is the autoload.php?

The autoload.php is auto generated by composer when dependencies are installed or updated. After running composer install in the root directory of the project you'll find a vendor/ directory which contains the dependencies and the autoloader. So in your code you would just need to include '/path/to/library/vendor/autoload.php' and you're good to go.
If you plan to use this library along with others in your own project, you just need to create a composer.json in whatever folder that should contain the vendor files and include the autoload.php from there. Like so:

{
	"name": "me/my-cool-library",
	"description": "My cool library",
	"require": {
		"php": "^7.4",
		"chillerlan/php-qrcode": "^4.3",
		"whatever/library": "dev-main"
	},
	"autoload": {
		"psr-4": {
			"Me\\MyCoolLibrary\\": "src/"
		}
	}
}