I entered Crypto market for 2 months and usually see the term "pump and dump", and some P&D groups. But i wonder they pump by selling and buying themself, so when price go up, who will buy it for them and they sell that for who???
With Blue-chip coins, someone could buy when it've pumped already because of good news. So with trash coins, who'll buy this pumped coin and why many PP&D group still live until now

You are asking a kinda pointless question.
There are many investors focused only on penny altcoins that are often pumped and dumped.
If there were no penny altcoin investors,the pump&dump method won`t work and no P&D groups would survive.Those investors don`t care about the "trash" altcoins.They care only about the short term profits.