Learn English – “These kids I tell you” or “kids I tell you” expression meaning

It's a sarcastic response.

Taken literally the sentence would mean the speaker isn't already familiar with the situation and wants more information. However it is rarely (these days never) meant literally, and is used sarcastically to mean the opposite. I.e. That the speaker is already familiar with the situation. Its meaning is similar to the expression, "You don't say?", where the speaker is acting like they don't already know, when in fact they do.

The difference between these two expressions however is that, "Tell me about it", has taken on a sympathetic tone over the years, indicating shared misery, whereas, "You don't say?", is an insult where you don't actually care what the person has to say.

There's little in the words themselves to indicate that difference. That difference is simply the connotations those expressions have taken on over time. Tone of voice is pretty critical here, because the difference between a sympathetic statement and an insulting one is simply a different inflection while saying the exact same words.

The whole phrase is definitely not some idiom, it's just normal use of the idiomatic phrase in this specific context:

I can tell that [something]...

is equivalent to "I know that [something] is true"

"Things below" in this case means the listed statements. You could replace that statement in the linked question with "I know that:"

The asker asks this to avoid answers to contain informations he knows already, so he lists his current knowledge before asking questions that depend on it.

In this case, below (not bellow) means simply "listed under this statement", normal physical below as "located lower".