From this article https://our.umbraco.com/documentation/Tutorials/Members-Registration-And-Logins/ you can see the Login macro code and able jump to and see the built-in UmbLoginController that the macro has a call to its HandleLogin action. You can look into it through your VS ide.

In case, you have migrated the db to V10 already, so all the Member data records are there, you need add LegacyPasswordMigration config:

"Umbraco": { "CMS": { ... , "LegacyPasswordMigration": { "MachineKeyDecryptionKey": "keyvaluefromtheV8siteweb.config_machineKey.decryptionKey" } } }