Yuttadhamo actually explains these concepts in his videos.
I may make mistakes but I will elaborate.
Firstly the concept of a beginning and ending of existence/the universe is a limited perspective from our minds as we do not understand reality as it is. Because of this we cannot comprehend existence existing without a beginning and end. I think it is classed as one of the imponderable things in Buddhist teachings. We could ponder it ad infinitum and never get to the point of knowing so it is a waste of time and does not lead to freedom,
Beings enter this world based on karma and dependant origination
As for your last question, just because the human race has grown in number that does not mean that sentient beings have grown in number. There are many sentient beings in this world, let alone other worlds, let alone other realms of existence, all of which are in a flux transitory state some of which have migrated to a human being existence in this world.