The easy installer will install /e/Oreo android 8, even over your actual stock android 10 !
( it will properly downgrading your system to android 8 )
If you want /e/Q android 10, you have to do it manually…
Official documentation for the s9 is not 100% up to date and accurate : if you have already stock android 10 installed, and you want to install /e/Q over it, don’t care about the flash android 8 vendor image additional step, it was for installing /e/Oreo over stock android 10…
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Edit : if you are a windows user,