Learn English – “How to help you” or “may I help you”

Both questions are asking how an individual acquired knowledge about the apples and can be used interchangeably in most circumstances. There is however a slight difference in nuance.

How did you come to know? emphasises questioning the mechanism by which the information was obtained. Did you search for the information, stumble across it or were given it? It also carries an implicit question of why did you want to know?

How do you know? still questions the mechanism and, as you say, carries the same basic meaning, but has no implicit why.

The meaning of the idiom can be used to say regardless of how horrible it may sound it's true, and it can also be used to say it wasn't so bad

P1: Their true intention was to steal from us!
P2: Really?! That's just horrible!
P1: Well, as horrible as it sounds, it was.

As horrible as it sounds, the 10 injections I had to get were painless.

It is usually used to speak about something bad, however, it can also be said sarcastically in a joking way for unbelievable good news

P1: As horrible as it sounds, I won a week at a beach resort!
P2: Oh no, that's awful, poor you! (sarcastic snicker)

The construction is as something as it sounds

good
bad
wonderful
desperate
stupid
boring
etc.

A similar idiom is: as something as it gets