Where is the mount point for a USB stick on Android 9?
You got it correctly, USB OTG sticks are mounted at /mnt/media_rw/[UUID].
Source:

// Use UUID as stable name, if available
    std::string stableName = getId();
    if (!mFsUuid.empty()) {
        stableName = mFsUuid;
    }
    mRawPath = StringPrintf("/mnt/media_rw/%s", stableName.c_str());


For FAT family of filesystems UUID is not the 32 digits long real UUID, but a serial number (8 digit long). So the mount point vold creates isn't static since Android 6. UUID (or serial number) of filesystem changes when you change USB stick or re-format it (unless you explicitly set the same UUID when or after creating filesystem). For static mount point you need to modify vold source code or mount filesystem manually. For more details see: How to change random filesystem label on USB OTG?

SD cards are also emulated form /mnt/media_rw/[UUID] to /storage/[UUID] but still the path is not static. See details here: How to move files to external SD card?