The standard installation is just a .phar (https://www.php.net/manual/en/book.phar.php) file. So probably just use the path ./wp-cli/php/boot-fs.php where wp-cli is your executable (guess). I know wp-cli does some weird exec steps, so it's worth reading though the code to find the correct bootstrapping point, aka boot-fs.php. You can also install packages globally with composer using the -g flag. Also, I would like to advocate using composer at all cost (it's free). If you're not autoloading your PHP files using composer's PSR-4 then you're not php-ing well!!! Python has pip, Node has npm, and we got composer. Do not be afraid like I once was; pretty much every language has standard-ish package manager. Shoot, even operating systems do ( Window = chocolaty, Mac = Brew, Fedora = pkg, Ubuntu = apt, etc.).

And on a more serious side-advocacy note: The composer scripts section is much better than npm's since you are allowed to have a script point to an array of commands with composer.json but not package.json. Inlining bash with ||&& and if statements stinks and is super hard to edit. If we had a command we needed to use a lot, never keep it in your notes; no, it gets put in the scripts section and generalized for every member of the team. Profit POV: Your scripts are now tracked, and you gave them pretty names everyone can reference and update without a DM.

https://github.com/CarbonORM/CarbonWordPressExample