Excel and Word both support real-time collaboration and sync, e.g., https://support.microsoft.com/en-us/office/collaborate-on-wo...

Sure, now. I'm responding to this from OP:

> Local-first was the first kind of app. Way up into the 2000s, you'd use your local excel/word/etc, and the sync mechanism was calling your file annual_accounts_final_v3_amend_v5_final(3).xls

Google docs is the gold standard here.