Teollistuneisiin
Teollistuneisiin is a Finnish grammatical form formed from the participle teollistunut (industrialized) with the illative plural suffix -isiin. The result, teollistuneisiin, marks movement toward or into a group described by the stem, or the target of an action in sentences that require the illative case. In practice it is used to refer to “the industrialized ones” or “the industrialized world” when the noun it accompanies is in illative plural, such as a destination or goal.
The form is most commonly found in phrases where a following noun is in illative plural, for
Phonology and spelling follow standard Finnish vowel harmony and consonant system; the illative plural suffix commonly
See also: Finnish grammar, illative case, participles and adjectives in Finnish, suffixation patterns in illative formations.
In summary, teollistuneisiin is a specialized, grammatically motivated form used to denote movement toward or inclusion