LXQt does not provide its own window manager but any arbitrary window manager can be used, most notably Openbox, KWin (the window manager of KDE), and Xfwm4 (the window manager of Xfce).
The current window manager is stored in section [General] of file session.conf,
[General]
[...]
window_manager=xfwm4
[...]
It can be chosen in configuration dialogue LXQt Session Settings of lxqt-session.