Classic workflows is a part of Microsoft Dataverse and it can be used to add no-code business logic and automation.
It is recommended to use Power Automate cloud flows instead of classic workflows. Cloud flows run asynchronously though, classic workflows can be run synchronously. Always consider to use Power Automate cloud flows instead of classic workflows. Another alternative is to create plug-ins, it requires writing code.
Workflows can be extened using code if no-code does not support what you want to achieve. Then a Custom Workflow Activity (CWA) can be created.