Die Menschen suchen...
- A group of people OR in some cases humans in general
Example (group of people): "What they want?" - "The people (die Menschen) are looking for better jobs."
Example (humans general): "When it's could outside, then the people (die Menschen) are moving to warm places."
- Humans in general
Example: "Humans (Menschen) are moving to warm places when it's winter."
I'd say, it's used like in english, think about when you use "the humans" and when you would use just "humans", in example, in german language, when thinking about the universe comparing humans and planets, I would always say:
THE humans on earth are fighting wars while THE humans on moon are peaceful.
If someone just asked me about good or bad things humans are doing, I will answer without article (so in general):
It's sad, that humans are fighting wars