Open the Shapes panel in the Sketch editor theme

Click on the Shape tool in the toolbar on the left to see the Shapes panel.

Drag the floating panel around the drawing canvas. Click on the arrow in the top right to minimise/maximise the panel.

Click on More Shapes to show or hide specific shape libraries.
Click on the Shapes tool to open this panel when using the Sketch editor theme in

From here you can also enable or disable the search feature and the scratchpad.
Show or hide the search and scratchpad features in the Shapes panel

See how to use a custom shape library in the Sketch editor theme.