I am currently using the Welcome Info panel from another website. The info panel has icons for each tab. For example, the VIP rank tab has a star icon, the Rules tab has a shield icon, and the Home tab has a house icon. I would like to use customs icons or different ones if possible. Is there a website where I can find these icons? I would also like to use them for other plugins as well. Thank you!

They likely found them on the internet or had them custom made.

I know there are icons somewhere in the game assets, but I have no idea how to get to them. Someone know?

https://github.com/OrangeWulf/Rust-Docs/blob/master/Extended/UI.md  has a probably exhaustive list of things in Rust that you can add to guis with a CuiElementContainer containing a CuiElement with a component CuiImageComponent, or maybe the simplified CuiPanel preset, but I use the former.

To display images sourced from a url, one can use CuiRawImageComponent, and often the plugin ImageLibrary to cache them and make them more responsive, but I've not used that yet.

I haven't been able to find any way of displaying the individual skinned icon/sprites that rust puts in its inventory guis, but I've seen people go around that with Url, and since skins are streamed they'll not be stored anywhere permanent.