A company that is not on QuickBooks Online joins Fynease through files: set it up as a CSV / Excel company, then import its data each month. It closes, reconciles, consolidates and reports like any other company; the one difference is that its adjusting entries are posted in its own system, because write-back is to QuickBooks Online only.
Drop several files at once. Each is recognised by its columns as a trial balance, general ledger, AR aging, AP aging, budget or customer sales file. You can change what was detected, and a file that is not clear asks you rather than guessing.
Or import one kind at a time: trial balance (which also builds the chart of accounts), chart of accounts, general ledger, AR and AP aging, budget (month columns), and customer sales.
Re-importing a trial balance offers to replace the period or merge while keeping your overrides. The importer includes export steps for QuickBooks Desktop, Xero and Sage. Imported data is kept permanently, because there is no source system to read it again from.