Liquid is custody by one centralized, opaque group of functionaries with transparecy over your coins and their backing still being there - until it's maybe gone one day. But it has quite good privacy properties. The liquid blockchain does incur a moderate cost per transaction, making it unsuited for small payments.

ECash is custody by groups or individuals with no transparency whatsoever. A mint could award you a billion Bitcoin tokens without owning a single UTXO. But eCash has excellent privacy properties.

Lightning is unconfirmed UTXOs that can get replaced a million times without incurring an on-chain fee. Getting the right one confirmed is the tricky part which makes it inferior to on-chain UTXOs.

Lightning is what currently bridges between the three, allowing you to pay LN invoices with eCash or Liquid wallets.