Zynthian detects LV2 plugins that are installed in the standard locations as well as in its own LV2 directory. After installing the LV2 you need to use “Search for Engines” in webconf SOFTWARE->Engines.
Zynthian’s LV2 search path is:
LV2_PATH="/usr/lib/lv2:/usr/lib/aarch64-linux-gnu/lv2:/usr/local/lib/lv2:/usr/local/lib/aarch64-linux-gnu/lv2:/zynthian/zynthian-plugins/lv2:/zynthian/zynthian-data/presets/lv2:/zynthian/zynthian-my-data/presets/lv2"
The search path is used for plugins and presets.