While editing a card, you can find a "Link" option in the quick-add menu (+ button).
Selecting this option will allow you to choose a card from your collection that you would like to link to, and will automatically create a markdown link for you. As a card link looks just like a normal markdown link, you can even edit the name of the link if you'd like!
Clicking on a link when in "Display" mode will then immediately open that card in Preview.
Backlinks are created automatically for you when you link one card to another (as above). In the linked card linked card, there will be a quotation mark icon. By clicking it, you can see a list of all the cards that have linked to the current card, and when the link was created. As with the rest of the platform, a single click on one of the items will open that card in Preview. Shift-clicking will immediately open the card in the Noteboard.