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I found an article online that tells you how do use your screen recorder
https://www.devicesfaq.com/en/record-screen/motorola-moto-g5-plus
From what I got from it, here’s a summary.
First, open your quick panel. This is where you adjust your brightness for example. If you don’t see the recording icon there already, click the edit button at the top right corner. It should look like a pencil drawing on something. Then, select the screen recording app, then it should appear on your quick panel. From there, activate screen recording. From there you should be pretty much good to go.