what are the differences between tokens and coins? 

why are they stablecoins and memecoins and not stabletokens and memetokens?

Coins are digital assets that operate on their own independent blockchain. Tokens are digital assets that operate on an existing blockchain network. While coins primarily function as a medium of exchange, tokens aim to offer a wider range of functionalities within a specific project's ecosystem.

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I agree with this but - ‘stablecoin’ is an exception because it’s a reference to $1 fiat, aka a physical $1 coin.
Stablecoins are actually tokens but having coin in the name is easier to comprehend and less nerdy.

Stablecoins are just stable in price, not necessarily pegged to 1$. RAI is an example of a non-pegged stablecoin.