I've created an authentication system with Laravel and the Steam API, but I'm clueless about how to retrieve data such as the inventory of the logged-in player so that I can display their skins, for example, in games like Rust or CS:GO. Does someone know of a way I could obtain information like this on my website? All the packages that exist seem to be outdated, and there's hardly any information available about it.
You will need to make a small wrapper but you can get a Steam user's inventory via this endpoint:
https://steamcommunity.com/inventory/{steamid}/{appid}/2?count=100
For example, here's a CS:GO inventory:
https://steamcommunity.com/inventory/76561198389912800/730/2?count=100
They return a JSON response so should be pretty simple to get working in your application. Steam has a pretty low rate limit so use it sparingly. More detailed answer here.