A screencast is a video recording of your computer screen, usually with narration. Your computer already has the software you need to make a basic screencast. Your computer probably already has built-in software for it. Once you're done, you can upload your video to YouTube or similar site for easy sharing (setting the permissions as you would like).
Use macOS's built-in screen-recording tools.
Press Command + Shift + 5 to open the screen capture controls.
Choose whether to record the entire screen or a selected portion.
Click Options and select your microphone if you want to record narration.
Click Record.
When you're finished, click the Stop button in the menu bar.
You can also access screen recording through QuickTime Player → File → New Screen Recording.
Apple: How to record the screen on Mac
Use the built-in Snipping Tool.
Open Snipping Tool and select Record, or press Windows + Shift + R.
Select the area of the screen you want to record.
Click Start.
When you're finished, click Stop.
Save the recording.
Microsoft: Use Snipping Tool to capture screenshots and video
You can also use Zoom to make a screencast. This is especially useful if you are already comfortable with Zoom or if your screencast includes a remote presenter.
Start a Zoom meeting.
If other people are presenting, have them join the meeting.
Click Share and select the screen or application you want to record.
Make sure your microphone is on. If you need to capture sound from your computer, enable Share sound when sharing your screen.
Click Record and choose Record on this Computer (or Record to the Cloud, if available).
Give your presentation as usual. Zoom can record your shared screen along with your narration and, if desired, video and audio from other participants.
When you're finished, stop the recording and end the meeting. Zoom will process the recording and save or provide access to the resulting video.
This makes Zoom a convenient option for interviews, conversations, or presentations where the speakers are in different locations.
Use the built-in Screen Capture tool.
Press Ctrl + Shift + Show windows.
Select Screen record.
Choose the entire screen, a window, or part of the screen.
Open Settings to turn on the microphone or device audio if needed.
Start recording.
Click Stop recording when finished.
Recordings are saved to your Downloads folder by default.
Google: Take a screenshot or record your screen on a Chromebook
For more control over screen recording, audio, webcams, and other sources, use OBS Studio. It is full featured, free and available for Mac, Windows, and Linux.