Can anyone explain to me how the isomers of double carbon bonding structures are formed ?

Once you have a >=< in the middle of a chain, the four atoms connected to that unit are fixed. There is no rotation about a double bond. So if you have CH3CH=CHCH2CH3, then the CH3 and CH2CH3 are on the same side (cis, Z) or opposite sides (trans, E). If you have -CH=CH2 at one end, the H's are the same, so there can be no same side/opposite side difference. Also the >C=O in -COOH does not show such isomerism.