How to read an USB flashdrive on a Samsung S9? I saw online that some OTG adapter is required. I want to know if there are other options requiring only an app or something.
An OTG adapter should be included with the Galaxy S9. If you insert the male connector into your device's USB port, and a flash drive into the adapter's female connector, the drive should be automatically mounted. You may not get any clear indication that this happened, however if you check storage settings, it should be there. It should also be visible and browsable in whatever filemanager you're using.
Don't forget to unmount the drive before physically disconnecting it. This is done in storage settings. Unmounting makes sure all cached transactions are flushed. Technically this should be done every time, but it's especially important if you've written anything to the drive. If you don't do this, you may lose data, as it's only written to cache, and never flushed to the drive.