If the organizational setting is configured, Outlook for Windows will respect that setting and make meetings online by default. OWA and Outlook mobile use an Exchange Online organizational setting (which can be overridden for individual mailboxes) to know if they should schedule online meetings.
As I write, I see the change in Microsoft 365 Apps for Enterprise build 13530.20218. It depends on updates to the Teams meeting add-in for Outlook and Outlook click-to-run (current channel). According to Microsoft roadmap item 66021, the feature will be available in January 2021. Office 365 notification MC230567 (updated January 20) brings the news that Outlook for Windows gains a similar feature. In mid-2020, Microsoft introduced new configuration settings to make online meetings the default when scheduled by OWA, Outlook for Mac, and Outlook mobile clients.
