Learn Portuguese (Portugal): A grua é muito alta e perigosa.

They are adjectives describing A grua (The crane), which is a feminine noun. In Portuguese, adjectives must match the gender of the noun they describe, so the masculine forms alto and perigoso change to their feminine forms alta and perigosa.

Why use é (from the verb ser) instead of está (from the verb estar)?

The verb ser is used for permanent, inherent characteristics. Being tall is a permanent physical feature of a crane, and being dangerous is treated as a general, built-in fact about this heavy machinery, rather than just a temporary condition.

Since grua is feminine, why doesn't muito change to muita to match it?

In this sentence, muito is an adverb meaning "very," and it is modifying the adjective alta. When muito acts as an adverb, it is invariable (it never changes its ending). It only changes to muita, muitos, or muitas when it is used to mean "a lot of" or "many" alongside a noun.

Can the word alta mean "high" as well as "tall"?