darin
in it
Refers to a previously mentioned place, container, or space and means 'in this' or 'inside it'.
Er öffnete die Schachtel, aber es war nichts darin.
He opened the box, but there was nothing in it.

in it / therein
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in it
Refers to a previously mentioned place, container, or space and means 'in this' or 'inside it'.
Er öffnete die Schachtel, aber es war nichts darin.
He opened the box, but there was nothing in it.

'Darin' (in it / therein) is a pronominal adverb and a compound of 'da' and 'in'. It replaces the combination 'in + pronoun' (e.g., in ihm, in ihr, in dem) when referring to a thing or a place.
Ich habe eine neue Tasche; darin ist viel Platz für meine Bücher.
I have a new bag; there is plenty of room in it for my books.
If 'darin' (in it / therein) refers to a concrete place or object, it means 'in this' or 'in it'. If it refers to an action, thought, or situation, it means 'in this respect'.
Das ist ein alter Schrank. Was ist wohl darin?
That is an old cabinet. I wonder what's in it?
In the abstract meaning, 'darin' (in that) often introduces a subordinate clause with 'dass' (that) or an infinitive construction with 'zu' (to). It often appears after verbs such as 'bestehen' (to consist), 'liegen' (to lie), or 'sehen' (to see).
Der Plan besteht darin, die Kosten um 20 Prozent zu senken.
The plan consists in reducing costs by 20 percent.
so that / with it
'damit' and 'darin' both belong to the words with 'da-' and refer to something mentioned earlier. The difference is the preposition. 'damit' means 'with this thing' and fits when you use something or when a verb requires 'mit'. 'darin' means 'in this' or 'in this matter'. It fits with rooms, containers, or when something lies within a topic or activity. The risk of confusion comes from the similar form. Ask yourself: Is it about 'mit' or about 'in'?

Morgen bekommt die Kundin eine Mappe, und darin liegen alle wichtigen Unterlagen.
Tomorrow the customer will receive a folder, and all the important documents will be in it.
Ich nehme den Mietvertrag zum Termin mit, damit der Berater ihn prüfen kann.
I’ll take the rental contract with me to the appointment so that the advisor can check it.