Garmin HRM1G ANT+ pair directly with Android for Strava use

I have an ANT+ certified phone. I don’t have a Garmin watch or any other device.

The Garmin HRM1G chest strap has good reviews and costs £14 (e.g. https://www.ebay.co.uk/itm/Brand-New-Garmin-Heart-Rate-Monitor-with-Strap-HRM-ANT-010-10997-00/113225044916?epid=2165709231&hash=item1a5cbd13b4:g:P08AAOSwKytZJAE~:rk:4:pf:0).

Would I be able to use this directly with Strava? (without any additional USB OTG dongle etc)

Yes, it should work.
Try this Ant Tester app. It will tell you if you phone actually supports Ant+. You might need to install the Ant radio services and plugins. That app has links to them.
https://play.google.com/store/apps/details?id=com.quantrity.anttester

Garmin strap works fine with my Android phone

Thanks. App says yes for built in ANT (firmware tag suggests Qualcomm chip).
Will give it a go.

Works fine. £12 well spent.