Learn German: Ich verstehe den Text besser, indem ich ihn zusammenfasse. - I understand the text better by summarizing it.

The conjunction indem expresses the method or means by which you do something. While English uses "by" plus an "-ing" verb (by summarizing), German builds a full subordinate clause with a subject and a conjugated verb. So, indem ich ihn zusammenfasse literally means "by which I summarize it" or "in that I summarize it".

Why is zusammenfasse written as one word here? Isn't it a separable verb?

Yes, zusammenfassen is a separable verb! In a normal main clause, you would split it: Ich fasse ihn zusammen. However, indem is a subordinating conjunction, which kicks the conjugated verb to the very end of the clause. When a separable verb is pushed to the end like this, its prefix and core verb stay attached as a single word.

Why does the sentence use ihn for "it" instead of es?

The pronoun ihn refers back to den Text. Because der Text is a masculine noun, and it acts as the direct object (the thing being summarized) in this second clause, it must take the accusative case. The masculine accusative pronoun for "it" (or "him") is ihn.