Select one of the tools (custom, ellipse etc) and go to the top left of control bar and select the fill or stroke panel. You can then select whether the fill and stroke are gradients or patterns or a solid color or set the fill to nothing and just have a stroke setting.
Change the fill or stroke via the appearance section of the properties panel found in the window menu. This panel does not need the shapes tool to be selected. All the same options for the gradient, solid color and patterns and nil are available for the fill and stroke color of the selected design.
You can also change the fill color via the layers panel. Go to the layer in the layers panel (window menu) and double click the entry and the panel should appear enabling you to modify the color. The panel varies depending on the color option currently used. You cannot change a solid color to gradient etc.
Go to the layer menu and layer content options and you can then change the gradient, solid color etc. You cannot change the type of option https://youtu.be/hq5_AsBbBnE
With the artwork selected, go to the swatches panel and click a gradient or solid color.
To fill the artwork with a gradient, go to the window menu and gradients and select a gradient and that will be used as the new fill. If you go to the right side menu for the gradients panel, you can set the default type used such as linear or radial or conical etc.
Display the patterns panel via the window menu. Select the design and click a pattern and that will then be used as the fill for the vector design.