I have read that fees when sending from segwit addresses are lower than fees when sending from legacy addresses. Is it true? If so, what kind of differences are we talking about?

Also, using bitcoin core's "bitcoin-cli addwitnessaddress 15......" command, do we have to do anything when spending funds received to a segwit address? Will all other sendtoaddress and sendmany commands work like before?

What about the estimatesmartfee and settxfee commands? Right now, when the network is quite busy we had fees of 20-25 EUR to send out from legacy addresses (to be confirmed in time). Do we have to set the same transaction fees (by settxfee) as before?

Is this a correct assumption that the same transaction initiated from a segwit address is ALWAYS lower in size than the same transaction initiated from a legacy address?