Busy and Marg each export differently, and neither writes a single file that carries everything the way a Zoho or Odoo backup does, so expect to bring masters and reports across as separate files.
Busy
Administration → Data Export Import → Export Data → Masters (Accounts, Items) → Excel. On Standard and Enterprise, the same screen can export XML too, when your version supports it. Basic exports Excel by report only; there is no XML option on that edition.
Marg
Reports → Management Report → Utilities → Data Import/Export → Tally/Busy Export writes ledgers, items and vouchers as two Tally-format XML files (a Masters file, then a Daybook file). Drop them on the same Pick screen and they are read like Tally's own Day Book. This is a different screen from ERP Bridger, which is Marg's own Marg-to-Marg transfer tool and does not write Tally-format files. Without the Tally bridge: Bridger → Export → Masters (ledgers, items) as CSV, then Reports → each report → Excel, and map the columns on the next screen.
For batch numbers and expiry dates on your stock, bring across Stocks → Current Stock's batch-wise report as Excel too, because the Tally export only carries a batch's name, not its manufacturing or expiry date.
What comes across today
Parties, items, your chart of accounts with opening balances, invoices, bills, payments and journals, from whichever export you bring. Stock movements keep their batch number, and manufacturing/expiry date when you bring the batch-wise report, so they show up in your batch and near-expiry reports.
What is deliberately not copied
Filed GST returns, IRNs and e-way bills stay with the portal that issued them. Bank-statement matches and attached files are not copied either. Busy's own print formats and user rights stay in Busy; Marg's scheme masters and rate lists stay in Marg.
After you drop the file
The same five screens follow as for every other source: matching columns, a dry run with a verdict on every record, then Import and Reconcile. The mechanics (and how undo and re-imports work) are in Import your data from Zoho, Odoo, Vyapar or a spreadsheet. Back to the full list of sources.