I have got some amount of BBS tokens staked and there have got some confusions related to it . How Claim , Restake and Extend works ?

I wanted to extend my tokens duration but that too without claiming the rewards but I wasn't sure whether my initially staked tokens will be extended only or the Rewards will be compounded automatically  and I have searched the Board here but couldn't get anything concrete on it so I opted to claim my tokens and simply extend my Stake period but I will wish to get some clarification on it for future staking .

Additionally I have that for same Base Gas the fee for extending is the highest I am not sure what's the reason behind that because in my thinking the fer should be same be it for extend od claim or stake .