A useStyle('libs') will include or load in your page a Web Asset with a type of CSS named libs that has been previously registered either dynamically or as part of the components joomla.asset.json file.
A Preset is a collection or profile of .css and .js Web Assets(definitions) that have been registered, normally in the joomla.asset.json file, and the usePreset command will load all of the .css and .js files defined in the preset definitionin your page.
I found this a good explanation of how to use Web Assets in a component that might be helpful in you understanding what is going on, https://www.dionysopoulos.me/book/concepts-webassetmanager.html
You will find a the joomla.asset.json files for each extension, including templates in the ../media/exetension_name directories