Modifying the sidebar menu icons

[jehoshua7]

I needed to add a new sidebar menu icon for a sitemap, so found one at https://fontawesome.com/search?q=sitemap&s=solid%2Cbrands , and the suggested code was

<i class="fa-duotone fa-sitemap"></i>

so added these lines to config.toml as follows:

[[menu.main]]
weight = 6
identifier = "sitemap"
name = "Sitemap"
pre = "<i class="sidebar-button-icon fas fa-duotone fa-sitemap" aria-hidden="true">"
url = "https://example.com/sitemap.xml"

it works fine. My question is, how does this work ? Is it the JS code or just the stylesheet code ? Also in this line

pre = "<i class=\"sidebar-button-icon fas fa-duotone fa-sitemap\" aria-hidden=\"true\"></i>"

what does the fas do ? How does that work ? Some lines have fab . Obviously a parameter that is parsed to the code.