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When you are surfing Internet, the traffic is not encrypted (plaintext). When you use vpn, the traffic become encrypted.

In order to read the encrypted traffic, you must have the key to unlock.

This is the reason why you are using vpn and most ISP won't able to detect. But with the current technology, this is doable if they have the intention.

Nothing that you do online is 100% safe, even if you are on onion (tor)