Seeking keyboard shortcut in QGIS for opening attribute table?

By default the F6 key is the shortcut to open the attribute table of the layer selected in the Layers Panel in Windows. If there is no default shortcut (which is the case in QGIS on MacOS) or you want to change the shortcut, you can go to "Settings - Configure Shortcuts" and select your own shortcut key combination.

In QGIS 3:

• F6 to open the attribute table
• Shift + F6 to open selected features only (Shift like Selected)
• Ctrl + F6 to open visible features only

Source: https://north-road.com/qgis-3-0-shortcuts/

Also, on the "Attributes Toolbar" you can click on the "Open Attribute Table" icon.