Third party file managers such as ES File Manager and RhythmSoft File Manager HD (others will be tested) do not show an USB OTG device.
It does not even get listed.
Only Android's pre-installed file manager does.
Android 7 Nougat onwards: hides USB OTG sticks from third-party file managers?
Question
Answer
As explained here How app can access files on USB OTG storages in Android 6.0 (API level 23) without root?, the accepted answer says about the change introduced from Marshamllow
In Android 6.0, any device that is not adopted is considered portable. […] Third-party apps must go through the Storage Access Framework to interact with files on portable storage; direct access is explicitly blocked for privacy and security reasons.
(Emphasis added)
Which means that the File explorer needs to use Storage Access Framework (requires root as indicated in the question on SO). This XDA post Files by Google adds support for browsing files on USB OTG drives says Files by Google has been updated with this capability (not tested by me).
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