https://fountain.fm/clip/uVT9Wp93cDLYKhPG5y22

Worth a listen, but the begining of this TL;DR part was confusing.

Script contains

1) Opcodes

2) Data

One must satisfy the INPUT locking scripts, but as *the sender* i.e. the one *making* the transaction, you DO NOT CONTROL the output locking scripts - that was already defined by the person who made/created the addresses you are outputing to ....

EXCEPT with OP_Return: the transaction creator/sender adds one additional output with a zero sats and Op_Return as the script. An OP_RETURN locking script is unspendable, so there's no ScriptSig that can unlock it.

Op_Return was added as the 5th Legacy Standard Script type MARCH 2014. All legacy scripts (pre-Segwit) are unlocked via the ScriptSig field.