Al-An'am: 6
"Have they not seen how many..."
"We established them in the earth" (مكنا له في الأرض): It means We granted them a place therein. Similar to it is the phrase "We gave him land" (arḍan lahu). From this is His saying: "Indeed, We established him in the earth" (Al-Kahf: 84), and "Have We not established for them a secure sanctuary?" (Al-Qaṣaṣ: 57).
As for "We established them" (مكناهم), it means We fixed them firmly therein. From this is His saying: "And We had established them in that in which We have not established you" (Al-Aḥqāf: 26). Because the two meanings are close, they are combined in His saying: "We established them in the earth [in a way] that We have not established for you."
The meaning: We did not give the people of Mecca the likes of what We gave to ‘Ad, Thamud, and others—in terms of physical stature, abundance of wealth, and reliance upon the means of the world.
"The sky pouring down upon them" (السماء مدراراً): The sky is the canopy; the water descends from it to the clouds, or the clouds themselves, or the rain. Midrār means pouring abundantly.
If you ask: What is the benefit of mentioning the creation of another generation after them?
I say: It is to indicate that it is not difficult for Him to destroy a generation and ruin their lands. He is capable of creating others in their place to inhabit His lands, just as He says: "And He does not fear the consequence thereof" (Ash-Shams: 15).
"But even if We had sent down to you a book written on parchment, and they had touched it with their hands, those who disbelieve would surely say, 'This is not but obvious magic.' And they say, 'Why was there not sent down to him an angel?' But if We had sent down an angel, the matter would have been decided; then they would not be reprieved. And if We had made him an angel, We would have made him a man, and We would have confused them with that which they are confusing."