TL;DR Summary: use OBS to add a gain filter that increases your microphone's volume. The audio from OBS is routed to VB-Cable so that you can choose it from any application you want to use your boosted microphone.
Step 1: install OBS. OBS (Open Broadcaster Software) is an excellent free and open-source application for Mac and Windows for video recording and live streaming. It lets you do a ton of stuff with video and audio like share your screen while still showing your webcam in a corner, smooth transitions (like the pros!) between showing yourself fullscreen and then showing your desktop, mixing audio from different sources, etc...
The installation itself is quite straightforward, just download the installer from the official site and follow their instructions, nothing special.
Step 2: install VB-Cable. VB-Cable is a free virtual audio cable that you're going to route sound to. Download and install VB-Cable. Then in any application that you want to use that sound you just choose VB-Cable as the source. For example in Zoom, Hangouts, Facetime, Messenger... just choose VB-Cable as the audio source device, instead of choosing your microphone directly.
Step 3: setup OBS to use your microphone. Launch OBS, click on the plus in the OBS Sources pane, and choose "Audio Input Capture". Then choose "Create New", give it a name you'll understand (like the model of your mic), and then choose your microphone as the source device.
Step 4: route your microphone to VB-Cable. Select your new audio source from the sources pane and click on "Filters". In that window click on the plus at the bottom and choose "Audio Monitor". Name it whatever you want (I usually leave it "Audio Monitor") and then choose "VB-Cable" as the device. What this does is send the audio from this device (your microphone) to the VB-Cable virtual audio cable. Don't close the filters window yet.
Step 5: add a gain filter to your microphone. In the same filters window from the previous step, click on the plus at the bottom again and choose "Gain" this time. You can just name it "Gain". This filter is where you'll boost your microphone's volume. Now all you need to do is slide the slider to the right and your mic's volume will increase. BTW you can add a ton of free filters this way to your microphone, for example noise reduction, compression, EQ, etc...
Step 6: That's it. Now just choose VB-Cable as the microphone source in any application. For example in the Zoom settings window, go to Audio and choose VB-Cable as the microphone. You can do the same anywhere on your computer, like in Facebook Messenger on the web, while on an audio or video call open the settings and choose VB-Cable as your device.
