Where does the Apple Bootcamp software store the drive definitions on the windows partition?
A drive is usually divided up into partitions. There is a table stored in the drive that contains this partition information. The Boot Camp software installed in Windows looks at this table to determine which partitions are bootable. APFS is fairly new and your Boot Camp software does not properly recognize bootable macOS volumes stored in APFS partitions. 

There is a workaround that can allow you to boot directly to your SSD from Windows. See the question: macOS partition disappered after installing windows 10 using bootcamp. I consider the accepted solution as a temporary fix. Apple may issue a Boot Camp update for Windows that will fix this problem. Or, Apple may  not.

Below are the steps to make this fix to your Mac. The commands need to be entered into a Terminal application window.


Create the new volume.  The diskutil command, shown below, shrinks the APFS container by 300 MB and creates the new 200 MB "Mac OS Extended (Journaled)" volume labeled "MacSSD via rEFInd". 

sudo diskutil ap resizeContainer disk1 119500M JHFS+ "MacSSD via rEFInd" 200M


After the command completes, restart the Mac.
Download the rEFInd Boot Manager software from this SourceForge web site. Enter the following commands to install the software to the "MacSSD via rEFInd" volume.

cd ~/Downloads/refind-bin-0.11.2
./refind-install --ownhfs /dev/disk0s3



  Note: The error message Could not set boot device property: 0xe00002bc was expected and can be ignored.

Configure the TextEdit application. Open TextEdit, then navigate to the "Preferences..." window. Uncheck all the "Options", as shown below. When finished, quit TextEdit.


Use the command below to navigate to the folder containing the refind.conf file. 

cd "/Volumes/MacSSD via rEFInd/System/Library/CoreServices"


Make a backup copy of this file.

cp refind.conf refind.conf.orignal


Open the file in the TextEdit application.

open -e refind.conf

Add the following lines to the end of the refind.conf file. You should be able to just cut and paste these lines. When finished, save the changes, then quit TextEdit.

#
# Added to make rEFInd silent. 
#
menuentry "MacSSD via rEFInd" {
    icon \System\Library\CoreServices\icons\os_mac.png
    volume "MacSSD"
    loader \System\Library\CoreServices\boot.efi
    ostype MacOS
    graphics on
}
timeout -1
hideui all
scanfor manual

Enter the following commands to modify the /etc/fstab file. This entry will instruct macOS not to mount the volume labeled "MacSSD via rEFInd" at startup.

sudo bash 
echo &gt;&gt;/etc/fstab
echo "LABEL=MacSSD\040via\040rEFInd none hfs rw,noauto" &gt;&gt;/etc/fstab    
exit



  Note: The character sequence \040 is just the octal representation of the space character.


When finished, restart the Mac.


Now when choosing Boot Camp from the Control Panel, you should get something similar to what is shown below.