It’s implied by context, there is no real word for it.

A: “I ate a hamburger.” B: “How was it?” A: “Overcooked.”A: “I ate at that new restaurant.” B: “How was it?” A: “Classy joint.”
A: “I ate a hamburger at that new restaurant.” B: “How was it?” A: “The restaurant or the burger?”

“It” in English is just a shorthand way of saying “the thing being talked about” (just as “he” and “she” are shorthand ways of saying “the person being talked about”). This makes it a pretty useless word since you either a) already know what’s being talked about (so why bother with the extra words?) or b) don’t already know (and all the its in the world won’t help you). The only reason we really use “it” in English is to fulfill the requirement that (most) sentences need to have an explicitly stated subject.

There is no such requirement in Japanese. The topic “the thing being talked about) is marked with the particle は and unless it (the topic, not the particle) changes there is no reason mention it again.

日本語は、毎日勉強しています。好きですが、難しいです。
This might sound strange if you learned that 「AはBです。」 means “A is B” since that would imply that the sentence above translates to “Japanese is studying everyday.” However, if you think of  「Aは…」 as “I’m talking about A right now.” then it works out fine. If you still find the sentence above confusing since it doesn’t say “I” anywhere (again, why bother if it’s obvious), you could also write
私は、日本語は、毎日勉強しています。好きですが、難しいです。　 …or…
私は日本語を毎日勉強しています。好きですが、難しいです。
This last one might not seem right, given what I just said. Since I started with 私は and never changed the topic, shouldn’t the second sentence also be talking about me? “I like myself but I’m difficult.” Maybe, but common sense and context makes it obvious that that is not the case. (We do this in English too.)

I ate a hamburger at that new restaurant.
Was it tasty?
Hard to say. I only got a few bites of the floorboards before they threw me out.