
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.

in that
Refers to a previously mentioned matter, circumstance, or activity and means 'in this respect' or 'in doing so'.
Die eigentliche Schwierigkeit besteht darin, alle zu überzeugen.
The real difficulty consists in convincing everyone.
'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.
there / with you
With a stated activity, dabei refers to an accompanying action that happens at the same time. Darin, by contrast, refers to the activity as the area or respect in which something applies. Dabei therefore works with a secondary action, while darin can refer, for example, to a skill in that area.

Lea übersetzt oft schwierige Texte und ist darin sehr gut.
Lea often translates difficult texts and is very good at it.
Lea übersetzt den Text und hört dabei Musik.
Lea translates the text and listens to music while doing so.