Where can I found all my vsti in dorico( ex: Kontak, omnisphere…)?

go to play-mode, command-4 on Mac, at the right side you have your VST-instruments-rack. Press ±button at the bottom to add a VST.
Per default, only VST3 version instruments are listed. If you want to use your VST2 version instruments, at the moment you have to apply a workaround, for inststructions look at this thread:

On the left side you see your players in tracks, click on the little triangles, there you find the popup-menu to select/change a VST for each player