Nel campo si vedono molti tulipani, ma il cipresso rimane il più alto.

Nel is a contraction of in + il (the preposition “in” + the masculine singular definite article “il”). So nel campo literally means “in the field.”

This si is the impersonal si, used to express general statements like “one sees,” “you can see,” or “there are seen.” Here, si vedono = “you can see” or “there can be seen.”

With the impersonal si, the verb agrees in number with the noun that follows. Since molti tulipani is plural, we use si vedono (not si vede). If it were singular—say una quercia—you’d say si vede una quercia.

Molti is the masculine plural form of molto used with countable nouns. Here tulipani is a countable, plural noun, so you need molti. (Use molto for singular countables or mass nouns: e.g. molto riso.)

Ma means “but,” introducing a contrast: there are many tulips but (yet) the cypress remains the tallest. If you used e (“and”), you’d just join the two ideas without that sense of contrast or surprise.

Rimanere means “to remain” or “to stay.” Saying rimane il più alto conveys that the cypress still or continues to be the tallest (perhaps unexpectedly), whereas è il più alto simply states the fact without that nuance of persistence.

In Italian, the relative superlative is built with definite article + più + adjective. Since cipresso is masculine singular, it’s il più alto (“the tallest”). The article is mandatory: without it you lose the superlative meaning.

Yes. If you want to specify the comparison group, you can say:

• Il cipresso rimane più alto dei tulipani.
  This reads “the cypress remains taller than the tulips.” Here dei is the contraction of di + i, marking “than the tulips.” Without that phrase, il più alto implies “taller than all the rest” from context.