Confused About the Exact Location of WDM Cabinet/Equipment

Dear friend!
WDM provides long-distance and high-bandwidth Ethernet channels for the access network. Currently, the WDM device is generally located upstream of the OLT device, and is used to connect the OLT to the router/switch of the Internet node, the router/switch of the core network (voice service), and the router/switch of the video service (IPTV).
If an Ethernet channel needs to be established between the OLT and the MDU/ONU, You can also use WDM equipment to implement long-distance transmission.
Thank you!