Where is the mount point for a USB stick on Android 9? I am working on IMX8QM with Android 9. I needs to configure usb stick on static mount point.

Once USB stick detected,
create sda1, after check I got:

mek_8q:/sdcard # df
Filesystem                1K-blocks     Used     Available    Use%     Mounted on
/dev/root                   1777588     1189664     587924     67%         /
tmpfs                       2918920     488         2918432    1%         /dev
tmpfs                       2918920     0           2918920    0%         /mnt
/dev/block/mmcblk0p12       249828      58936       190892     24%        /vendor
/dev/block/mmcblk0p14       9086336     967628      8118708    11%       /data
/data/media                 9086336     967628      967628     11%     /mnt/runtime/default/emulated


/dev/block/vold/public:8,1  3918844     1311400     2607444    34% /mnt/media_rw/2A61-DD07
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?