Learn Russian: Потолок в комнате высокий.

In Russian, when you want to say 'The [noun] is [adjective]', you put the adjective after the noun to act as the predicate. If you said высокий потолок, it would usually just mean 'a high ceiling', but placing it at the end clearly means 'the ceiling is high'.

Where is the verb 'is' in this sentence?

In Russian, the present tense of the verb 'to be' is almost always left out. You simply state the subject (потолок) and its description. The literal translation is just 'Ceiling in room high'.

The current lesson is about prepositions like под (under). How would that look if we talked about something under the ceiling?

To say a lamp is located under the ceiling, you would use под plus the instrumental case of потолок, which becomes под потолком. In our current sentence, we are just saying the ceiling itself is in the room, so we use в plus the prepositional case (в комнате).

Why does высокий end in -ий rather than the standard masculine ending -ый?

It is a masculine, nominative adjective matching the masculine noun потолок. It uses the -ий ending instead of -ый because of the Russian spelling rule that says we cannot write the letter ы after the letter к.