I have HP Proliant DL380 Gen9 and on it VMware ESXI. I want to find out detailed info about the device, specially about harddisk bay and connectors, so to be sure if i can add extra harddisk expansion bay. I browsed HP ILO, remote console, vSphere Client but i can't get needed info. How do i get detailed hardware info without opening the device?
This information will be found in the documentation for the server, not on the server itself.
If you do not have the original configuration for the server handy, you should be able to plug the serial number into the HP support site to find the original config, and then by looking at the capabilities for that model, you should be able to determine what capabilities can be added with extra hardware.
This is the standard way of determining the upgrade capability of a server.
There will not be a place in the server itself that will say "Hi, I can accept an Acme Drive Tray and a Widget Connectorinator here".