Monarch
Export transactions rather than a report. The file's header reads Date, Merchant, Category, Account, Original Statement, Notes, Amount, Tags. Monarch writes spending as a negative amount, which Footnote flips on the way in so that spending is spending everywhere.
The Original Statement column is kept as a fallback name, so a row with a blank merchant still lands with something readable on it rather than as an untitled charge.
YNAB
Export the register, not the budget. The header reads Account, Date, Payee, Category, Memo, Outflow, Inflow, and Footnote treats Outflow as spending and Inflow as money in. Category Group/Category is read as well, so a YNAB category survives the move.
Dates are accepted as M/D/YYYY, YYYY-MM-DD or D/M/YYYY, which covers the non-US exports.
Copilot
Copilot is the exception: there is no CSV worth reading, so Footnote reads the app's local database file directly and takes the name overrides you have made along with the original merchant strings.
What happens on import
Rows are matched to your connected accounts by the account name in the file. Unmatched names become their own accounts rather than being silently merged, because merging two accounts that only look alike is the kind of mistake that is very hard to find later.
Imports are capped at 100,000 rows and 40 MB per file, and an import can be removed again without touching anything else. Comparisons: Monarch, YNAB, Copilot.