Where is the pipeline (opportunities) board like in GoHighLevel, and can I customize the pipeline cards?

The pipeline is under Contacts. At the top you have saved views, and each view can be shown as a table or as a pipeline (board). To change one, create a view or right click an existing view and switch it between pipeline and table.

Pipeline cards are customizable. You can add whatever custom fields you want, choose what shows on the card (for example last call date), and click to edit contact fields directly on the card. Each saved view can have its own columns and filters.