They, collectively is a reference to humanity. And in one particular generation, as David was referring to his own generation, so too, when the last generation comes onto the scene, then there will be a time during those last several weeks and months of human history that no person who remains alive will have any good that can be discovered among them.
There will be a remnant that is in hiding and under the protection of God, but as for all else, the overwhelming population will be degenerate and evil to the core.
Likewise, all of humanity is examined and all of humanity from Adam to the last person born, will be found to be corrupt.
None among humanity is good. None among humanity is perfect. None among humanity has the wherewithal to correct the injustices and flaws of themselves let alone all of human history.
The opposite of good is evil. If a thing is not good, then it is bad. The opposite of acceptable is unacceptable. And when a thing is unacceptable, then it is … well, unacceptable. That should be obvious.
Filthy means defiled. All together means everyone, with no exceptions.
Not even one, means just that. All of humanity from Adam to the last person who will ever be born, there will be not even one single person who will be excluded from this category of depravity.
From the previous verse, the children of men excludes Jesus Christ, who was not a child of a male father. He was born of the seed of the woman, excluding the man.
This then makes it clear that there is no person among humanity that is perfect. There is not even one single person who is not evil and filthy due to sin.
There is not one single person born of the children of man, who is qualified to pay the price of sin and death.
Quoted in Rom. 3:10-12, this is the source of the principle of the universal depravity of man. Depravity means immorality, corruption, wickedness, evil, decadence, degeneracy and so forth.
No good, means just that. There is no dignity of man, nothing that would indicate that man is innocent in any fashion. There is nothing that would suggest that man is fundamentally good, but just went astray accidentally.
There is no room for meaning well, for sincerity, for good intensions. Absent good, there is nothing that can mitigate mans depravity.
The doctrine of the total depravity of man, describes the fallen state of humanity as a result of the original sin and that the entire human race lacks the ability to remove that fallen state, apart from the work and grace of God. The doctrine states that man is inclined to serve their own will and desires while rejecting God. Even the good that man does, called human good as well as religion, are insufficient and lacking in their intents, because they originate from the imagination and invention of man and are done for mans own ego and glory, not Gods glory.
Mans good is self-serving and therefore a furtherance of mans depraved state.
Mans religion is likewise self-serving and therefore a furtherance of mans attempt to remove his depraved state, by excluding God.
Therefore, all human good, and all human sin are faulty and unable to correct mans fallen state.