Well, depending on how you look on it, it might not always be on the blockchain. Lightning network transactions are Bitcoin transactions that don't get immediately broadcasted to the network and added to a new block. This allows the parties that have a Lightning channel to save on fees by making many transactions between themselves and then only making one transaction on the blockchain. So Bitcoin in Lightning Network doesn't exist on blockchain until the channel is closed.
Then there are things like Bitcoin custodians. They could be running at fractional reserves and lending more Bitcoin than they have, effectively creating virtual Bitcoin. This does not exist yet, but might become more widespread in the future.
wrong
bitcoins never leave the blockchain
every coin ever mined stays on the bitcoin network
lightning network uses a locked reference of bitcoins that are LOCKED on the bitcoin network
where by the locked UTXO is then referenced to peg millisat balance on the lightning network at a ratio (thats not a hard rule and can be manipulated so be warned) of 1sat:1000msat
the lightning onion payments are measured in millisats not bitcoin
millisats(msat) are a unit not recognised by the bitcoin network
lightning networks pegging mechanism is not secure. LN does not have a network wide audit system or consensus to secure value. thus its not a network that should be thought of a secure trusted network for long term store of value
its the whole purpose of why blockchains are such a great invention because smart contracts were weak. all cypherpunks knew this.
with recent issues with many sidechains and subchains that messed with their pegs. it should be best practice to make people aware not to trust other networks that say they are transacting bitcoin when infact they are not the bitcoin network and using tokens/units of a different name/measure.
fine use the other networks as a short term niche use for a certain reason as long as you learn about the risks and accept the risks. but dont trust them as the long term store.
same goes for using middlemen services where you send them bitcoins to their bitcoin network address and they give you some other database balance displayed on their website to play with. that website balance on their site are not real bitcoin. they are just a balance on a database that supposedly represents balance they owe you
bitcoins security is the very reason bitcoin is great, be aware of the risks of services and other networks that pretend to offer you bitcoin off the network