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Typically open banking refers to a Banks provisioning of Oauth style APIs that allow users to share banking permissions with third parties. So a third party provider (eg payment company, budgeting app etc) can grab your statement data and trigger payments on your behalf just like you would from your mobile banking app.

ACH & SEPA are payments rails, which facilitate the interbank transfer of money. So for example, the app you have given permissions to make transfers to will request a transfer through the open banking APIs to your bank, then the bank itself will use the payments rails to process the transfer.