> Can you explain how this works? This sounds like you were familiar with Japanese, not "started studying".

An important piece of context here (for anyone not versed in Japanese) is there there's effectively two sets of characters you need to learn, kana and kanji. The former is sort of an introductory requirement if you plan on learning, and the latter is something that takes most people years.

Kana is (for all intents and purposes you care about as a beginning language learner) split into two sets - hiragana and katakana. These are phonographic and cover specific mora (similar to a vowel), and both cover the same sounds. The corresponding hiragana and katakana often look similar, e.g. (ni) に and ニ, some are identical in both such as (he) へ, some are totally different such as (tsu) つ and ツ.

There's not that many of them, and you can probably learn them over the course of a week if you study diligently. Hiragana is the more important of the two to learn, and katakana is used somewhat similarly to how we would use italics or bold, for sound effects, for foreign loanwords, and similar.

Kanji is the other set of characters, and it's one of the tougher aspects of learning Japanese. ~800 kanji make up the 90th percentile of usage, but then you need another 1300ish to get to the 99th percentile, plus any domain specific ones. To further complicate things, there are multiple "readings" for quite a few kanji. Most kanji are imported Chinese hanzi, and many retain the original (or something close to) Chinese meaning as one of the readings, and a Japanese-specific one for another. The Japanese specific ones will often have hiragana attached.

All kanji can be written in kana. It's not generally done in practice for a variety of reasons - there are plenty of kanji that have the same pronunciation, it takes up more space, it's slower, etc. If I write out "Please buy some sake" (さけ) in hiragana, you won't know if I'm referring to the drink (酒) or the fish (鮭) without further context. Content aimed at people up through high school will frequently have furigana - the kana used for the kanji in question - above kanji, as will most content that uses rarer or domain-specific kanji in a situation where you can't expect the person to be familiar with it.

But when it come to writing beginner textbooks, they can keep things pretty focused, provide enough context, etc., to make it pretty feasible. There's lots of great electronic dictionaries now, so you can pretty easily look words up if you know the kana, etc.

>But how does this work without full immersion?

I think you might be overthinking it a bit - you need to do a little up front before you start working on vocab/grammar/comprehension/etc., but after that it's really just look at word -> look it up if you don't understand it -> figure out the sentence via context and dictionary results -> internalize -> repeat.

Cliff notes: Learning the kana, specifically hiragana, is basically pre-work to start learning Japanese in this sort of situation. I don't know if it's the right approach for everyone - some people might care more about learning some conversational basics before they go on a trip, or just generally need more immediate progress to stay motivated, but it is a method that is constantly self-reinforcing and likely works quite well for the people that can handle that sort of approach.