Fiddler does not run "on" mobile devices like phones; it runs on desktop PCs running Windows or Linux (and sometimes Mac). It can capture traffic from mobile devices and other PCs by configuring those other devices to point their proxy settings at the PC running Fiddler.
It does not attempt to "sniff" traffic from devices without configuration of those devices (e.g. like a promiscuous mode packet capture tool) although it can import unencrypted requests from PCAP files generated by such sniffers.
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