Each network has its own rules and native coin. Ethereum network uses ETH, Bitcoin network uses BTC. Transactions happen inside the network and they don’t naturally talk to one another.
Stablecoins are pegged to $1 regardless of which network they’re on. A stablecoin can exist on multiple networks, but each version is separate. USDT on Ethereum is ERC-20, for example. The $1 is the same value as a stablecoin on any other network but you can’t send ERC-20 to the Bitcoin network.
A bridge locks your token on one network and “mints” a wrapped version on another.
If you want to move USDC on Ethereum to USDC on Avalanche you use a bridge (or an exchange) to lock your Ethereum tokens and create Avalanche tokens (sorta like escrow or a lock that requires two keys). You can bridge back, the Avalanche coin is burned and Ethereum coin is unlocked. Like having a claim ticket for your car at valet.
Centralized stablecoins are backed by real assets like fiat and you’re putting your faith in a regulated entity and some folks feel more confident in that. Decentralized is putting your faith in the code but you can see the transaction on-chain so some folks feel more confident in that.
Some issuers release their coins on multiple networks (like USDC) but they reside independent of one another despite sharing the same reserve.
excuse typos, walking dog, typing with one thumb