this is considered to have asymmetrical carbons :ch3ch2ch(chlorine group is connected on bottom)Ch3
why isn't this one: ch3ch2ch (methyl group attach to it) ch3?
By convention, we look at the carbon chains, and carbon sidechains when discussing symettry. If the second were this, CH3CH(CH3)CH(CH3)CH3 we would say it was symettrical.
To determine whether a molecule has asymmetrical carbons, you need to identify carbons that have different substituents attached to them. In the first example you provided, "CH3CH2CH(Cl)CH3", there is a chlorine (Cl) group attached to one of the carbon atoms, while the other carbon atoms are attached to hydrogen (H) atoms. Since the substituents are different on at least one carbon atom, this molecule is considered to have asymmetrical carbons.
In the second example you provided, "CH3CH2CH(CH3)CH3", all the carbon atoms in the molecule have the same substituent attached to them, which is a methyl (CH3) group. This means that there are no different substituents on any of the carbon atoms, making this molecule symmetrical rather than asymmetrical.