Learn Hardcore Italian: Metto la scorza di limone nella teiera e il profumo è così intenso come quello dei fiori di menta. - I put lemon peel in the teapot and the scent is as intense as that of mint flowers.

In Italian, singular countable nouns generally need the definite article: la scorza (“the peel”). Dropping the article sounds unnatural unless in very terse labels.
Also, scorza specifically means the fine outer zest of citrus (used for aroma/flavor), whereas buccia is the thicker, less fragrant skin. Here, we want the zest, so la scorza di limone is more precise.

Why is nella teiera used? What’s happening with in + la?

Italian combines many prepositions with the definite article. In + la becomes nella, meaning “in the.” So nella teiera means “in the teapot.” Saying just in teiera is uncommon because we usually specify “in the teapot” rather than speaking in the abstract. Using alla teiera (“to the teapot”) would change the meaning to “toward” or “at” the teapot, not “inside.”

Why do we use il profumo here? Can we omit il?

When you talk about a particular scent (the one we just set up), Italian uses the definite article: il profumo. Omitting il (just saying profumo) would sound ungrammatical in a full sentence. Definite articles are typically required before nouns in Italian, especially when you’ve already introduced them.

How does così intenso come quello work? What structure is this?

This is a comparative of equality, using così ... come, equivalent to English “as ... as.”
Structure: così + adjective + come.
So il profumo è così intenso come quello dei fiori di menta = “the scent is as intense as that of mint flowers.”

What does quello refer to, and why not repeat profumo?

Quello is a demonstrative pronoun standing in for profumo (a masculine singular noun). Instead of repeating profumo, we say quello (“that one”). It makes the sentence smoother: come quello dei fiori di menta = “as (intense) as that (scent) of the mint flowers.”

Why is there dei fiori di menta? What is dei?

Dei is the contraction of di + i, the plural form of the partitive/genitive article, meaning “of the.” Here it links quello and fiori di menta, so quello dei fiori di menta literally means “that (one) of the mint flowers.”

Could we say tanto intenso quanto instead of così intenso come?

Yes, tanto ... quanto is another valid comparative of equality. You could say:
Il profumo è tanto intenso quanto quello dei fiori di menta.
Both forms are correct; così ... come is more colloquial, while tanto ... quanto is also widely used.

Why is metto in the present tense? Would ho messo work here?

Metto (present indicative) describes a habitual action or a step in the process (“I put the peel in when making tea”).
Ho messo (present perfect) would emphasize a single completed action in the past (“I have put the peel in”). If you’re narrating just what you did, ho messo works; but if you’re describing your usual way or an immediate action, metto is more natural.