Can I share customers across stores or keep them separate?

Both. Magento has a customer account scope setting (Stores > Configuration > Customers > Account Sharing Options): set to Global for one account across all websites, or Per Website to isolate. Most multi-region setups want per-website isolation (so a UK customer can’t accidentally order at US prices), while same-language sibling stores often share globally. Switching this setting after launch is painful — we always decide it upfront in the audit.