[spiralganglion]

A bit of both.

Lots of designers/devs will use existing assets to save time. They often pull materials from asset packs that other designers publish, as you mentioned (The Noun Project also comes to mind).

On the other hand, a lot of designers make it a point of pride to make their own assets from scratch. I'm certainly in the later camp, though I have friends in the former. It's nice to be able to say "I made everything on the site", but there's a definite cost in time and effort and pain.

[Teichopsia]

That's a nice project. I'll have to take a deeper look into it. Is there a central place where these types of projects pop up?

[spiralganglion]

Most of the things I'm aware of just came up on HN, reddit, twitter, or in podcasts.

The best approach I've found is just to follow popular designers (on twitter, podcasts, etc). When they talk about something cool, dig in and learn who made it, and follow that person to see what cool things they talk about. Repeat.

Maybe start with @brad_frost and @vlh.

[Teichopsia]

Added. Thanks.