verbs - Can you explain which word is connected to the word 'left'? What is the grammatical construct of the bold?

I am trying to understand the bold part of this quote from "Sky-high house prices in the most desirable cities are holding back growth and jobs":

As transport costs started to fall at the beginning of the 20th century, many of the manufacturing firms clustered in cities in developed countries left in search of cheaper land and labour.

• Which word is connected to the word "left"?


• Who left? The "developed countries", or "the manufacturing firms"?


• The word "left" is after "countries". How can you know the "firms" left?


• What tenses are involved? "firms clustered" and "developed countries that left" suggest past tense.

Answer

As transport costs started to fall at the beginning of the 20th century, many of the manufacturing firms clustered in cities in developed countries left in search of cheaper land and labour.

Many of the firms left. ("Many" = subject; "of" = preposition; "the" = definite article, which is a kind of determiner; "firms" = noun, the superset that the subject is part of; "of the firms" = prepositional phrase acting as an adjective; "left" = verb)

What kind of firms? Manufacturing firms. ("Manufacturing" is a gerund acting like an adjective.)

How were the "manufacturing firms" organized? They were clustered. ("Clustered" is a past-participle at the head of an adjectival phrase.)

Where were the "manufacturing firms clustered"? They were in cities. ("in" is a preposition; "in cities" is a prepositional phrase that acts as an adverb.)

Where were the "cities" the "manufacturing firms clustered in"? They were in developed countries. ("in" is a preposition; "developed" is a past-participle that describes "countries"; "countries" is a noun; "in developed countries" is a prepositional phrase that acts as an adjective.)

Why did "the firms" leave? They were "in search of" . ("in" = preposition; "search" = noun form of a verb; "of" = prepositional phrase that completes the idiom; the whole phrase acts as an adverb.)

What were "the firms" seeking? They were "in search of" cheaper land and labour. ("land" = object; "cheaper" = comparative adjective describing both "land" and "labour"; "labour" = "object"; "and" = conjunction; "land and labour" = compound object; "of cheaper land and labour" = prepositional phrase that acts as an adverb.)

What is the "cheapness" of the "land and labour" being compared to? It is implied that the cost "of land and labour" (in the places the firms moved to) was cheaper than the cost "of land and labour" (in the places the firms moved from).