Set up for Online Meetings and screen sharing on a Mac varies from the ridiculous (Safari) to the sublime (Chrome and Edge). First, the ridiculous.
Safari
Safari is the default browser on a Mac and for all intents and purposes, it's a good choice. Apple has put a lot of effort into making Safari secure, and ensuring it protects users and user data. That said, it has some limitations when it comes to surfing websites and using some of the web-based tools companies make available. Screen sharing is a perfect example of this.
Safari severely limits what can be screen shared. In fact, it only allows a user's primary screen to be shared -- you can't share additional or external monitor screens, you aren't able to share select apps or even different browser tabs. In that respect, Safari is a bit too limited.
In addition, Safari is a bit restrictive, and restricted, when allowing a website to use your microphone and camera. If you don't allow it when initially asked, you kind of have to dig down into your Preferences. So, when presented with this when accessing an Online Meeting:
Make sure you allow the page to use your microphone and camera.
Chrome and Edge
Chrome and Edge, as they both use Chromium as their base, are a bit more flexible and a bit more forgiving when screen sharing. In fact, when you click the Screen Sharing button in an Online Meeting, you're presented with this:
As you can see, your options are quite extensive: you can sharing entire screens (primary as well as any external monitors), specific application windows or even different browser tabs.
Firefox
Firefox is a bit more flexible than Safari for Screen Sharing, but it's not quite as flexible as Chrome and Edge. When clicking the Screen Sharing button you're actually presented with a dropdown where you select what you want to share. You DO have the ability to share various windows and screens: