Only the Core set is “free” (on loan, so to speak).
Classic/Legacy cards are not free, you can either craft them, or find them in Classic Packs.
Each Classic card has a Legacy counterpart, craft or open one, and the other will be added to your account. The reverse is also true; disenchant a Legacy card and you’ll lose access to its Classic counterpart.
Legacy cards can be used in Wild Format, while Classic cards can only be used in Classic Format.