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Newb: What can be done out of the box?

***I’ve played around with Untiy in the past.
What does the unreal engine offer out of the box?
If leverage their supplied assets can you get a character walking around a “world” with basic AI with now programming?

If so how “easy” is it to add custom AI logic to like “monsters” via C++?

Your best bet for “out-of-the-box” support is going to be in the Learn tab of the epic launcher. Best place for hitting a broadstroke on everything is in the Content Examples project. Though I don’t know if you’ll find any AI examples in there, you can always check the UnrealEngine channel: Unreal Engine - YouTube

Adding AI is easy, though depending on what specifically you’re intending, it’s easier to do so using Blackboard/BehaviorTrees.
https://docs.unrealengine.com/en-US/Engine/AI/BehaviorTrees/QuickStart/index.html

Everything you listed is fairly easy to do with the engine as-is, no plugins or extra downloads needed.

I would also add to what @NotSoAccurateNo1 showed, the Action RPG sample, gives a complete game example which can run also at mobile. You got a main character, weapons, potions, skills, enemies with AI all there for people to study.