The manual process is available on the Direct Banking Account page. In the action ribbon you can start the import transactions action.

For both imports (scheduled or manual) you will only import the new statement lines available after the registered last transaction id. This id is visible in the account page.
In the first attempt of synchronization this field will contain only zeros (00000000-0000-0000-0000-000000000000) .

The financial transactions will be imported automatically (for Job Queue settings check Job Queue Entry). The job queue imports the bank transactions, creates payment reconciliation lines (W1) or puts all the transactions in the Bank/Giro journal (NL).
If you use the local functionality from Microsoft Dynamics 365 Business Central, without the Direct Banking extension, you have to import the electronic bank statement for all your financial interactions manually.
You should reconcile your bank, receivables, and payable accounts on a regular basis by applying payments recorded in the bank to their related open (unpaid) invoices and credit memos or other open entries in Business Central.