Paid Zoom plans officially support joining multiple meetings at once, yet this is also possible on a free account if you don’t mind some of the quirks.
One meeting can use the desktop app—or a browser—but the others must all join from within a web browser. Using a browser can actually be a challenge as the native app tries to automatically launch whenever meeting links are loaded. So you may need to maintain a separate browser or profile which doesn’t auto-open meeting links in the desktop app. The process will also include clicking the big, blue “Launch Meeting” button then waiting for the smaller “Join from your browser” link to appear.
With everything connected managing the audio can be a chore since they’ll play simultaneously and your mic will pick up the others’ audio too. Muting yourself and using a headset are a must. It may also help to assign the other meeting’s audio to a separate speaker, such as those connected to another laptop/screen where the other meeting is on display.
For even more control over the volume of the meetings there is BackgroundMusic for Mac and Windows’ own Volume Mixer which can set different levels for the native app and your browser(s). Putting each meeting in a different browser is an option if you don’t use the desktop app at all or want to connect to more than two meetings at once.