Security is not my main concern because I always keep my pc and phone protected with anti virus and as a tech guy i am aware of it.
Honestly... it should be. Regardless of how good you think your antivirus is, there will always be malware that it won't/can't trap.
1) My main concern is , I like online wallet like bitcoin.com, trustwallet etc.. where they provide private keys and 12 words secret phases. Can they have still have custody of my coins anyhow? Is coins stored on their server?
Online wallets and security are two very different things... and in almost all situations are mutually exclusive.
You would be wise to heed the advice of o_e_l_e_o and the others.