darunter
among them
Indicates that something or someone is in a group that was mentioned previously; among others, included.
Es kamen viele Gäste, darunter auch einige alte Schulfreunde.
Many guests came, among them several old school friends.

among them / underneath
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among them
Indicates that something or someone is in a group that was mentioned previously; among others, included.
Es kamen viele Gäste, darunter auch einige alte Schulfreunde.
Many guests came, among them several old school friends.

'Darunter' is a pronominal adverb made up of 'da(r)' and the preposition 'unter' (under). 'Da(r)' refers to something that was previously mentioned in the sentence or context.
Auf dem Tisch liegen viele Bücher, darunter mein Lieblingsroman.
There are many books on the table, among them my favorite novel.
The context shows the exact meaning: If 'darunter' refers to a group, it means 'included/among them'. If it refers to a place, it means 'underneath'. If it refers to a number, it means 'less than/below that'.
Es gibt viele Apfelsorten, darunter süße und saure.
There are many apple varieties, among them sweet and sour ones.
In the meaning of 'eingeschlossen' (included), 'darunter' often introduces a subordinate clause or an apposition to give examples from a previously mentioned group. It then usually comes after a comma.
Viele europäische Städte sind sehenswert, darunter Paris, Rom und Prag.
Many European cities are worth seeing, among them Paris, Rome, and Prague.
The combination 'darunter leiden' (to suffer from it) is a fixed expression. 'Darunter' refers to the cause of the suffering, which was mentioned earlier in the conversation or text.
Die ständige Kritik verletzte sie, und sie litt sehr darunter.
The constant criticism hurt her, and she suffered greatly from it.
under
'unter' and 'darunter' can both mean a position lower than something else. They can also express that someone or something is part of a group or is below a limit. The most important difference is grammar: 'unter' is a preposition and comes before a noun, for example 'unter dem Tisch' (under the table) or 'unter den Gästen' (among the guests). 'darunter' is an adverb and usually refers to something that has already been mentioned. Learners easily confuse the words because they sound similar and cover similar areas. In practice: if a noun comes after it, 'unter' usually fits. If you are referring back to something mentioned before, 'darunter' often fits.

Im Karton liegen viele alte Fotos, darunter auch ein Bild von meiner Oma.
There are many old photos in the box, among them a picture of my grandma.
Die Katze schläft unter dem Sofa.
The cat is sleeping under the sofa.