How to capture clear text traffic from devices that arent my computer

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When I browse the internet on my computer and login to a basic HTTP web app, I notice WS captures the clear text traffic and presents it neatly. However I noticed I can only capture it when I use my specific computer. For example, when I use my iPhone to login, or my buddy logs in from his computer or phone, that same clear text traffic doesn't show up on my computer's wireshark. All devices are on the same network...I'm sure this is a basic level question, i'm new to WS so i appreciate any help...Any thoughts or explanations? Been struggling with this for some time now.