Unity has so many built in goodies but they aren't always obvious or easy to find by chance. Google is a really good way to pick up some know how on using the tools that Unity has to offer. I imagine there are many options for other engines too and again, googling the task you want to achieve can be a speedy resolve.
Unity tools I use a lot (These are all found through Package Manager, Under the Unity Registry drop down.)
ProBuilder,
Terrain Tools,
TextMeshPro,
Post Processing,
Recorder,
PlayerPrefs Editor,
AI Navigation.
From the Unity Asset Store some handy free tools (Although I only ever really use Font Nation a lot.)
For paid assets I have purchased: (But I don't even use these much!)
A great website I use for audio, sound-fx, music and similar is pixabay.