There are two ways to add issues to a board:

Add an Existing Issue from the Backlog

To add issues to a board, locate the issues in the Backlog column and drag them to the appropriate column/swimlane.

Dragging and issue from the backlog

For Sprint PI Planning boards, issues can only be added to their corresponding team's row - so an issue from Team A's backlog can't be added to a sprint for Team B.
As you add issues to the board, you may see some colorful arrows appear between cards - these are dependencies between the issues. See Managing Dependencies.

Searching the Backlog

You can search for issues in the backlog using a text search or JQL query.

Backlog search

Filtering the Backlog

On Sprint PI Planning boards, you can filter the backlog to see only issues for specific teams. To do so, open the Teams menu and select the teams you want to see.

Filter to see only certain teams

This will also hide the other teams from the board. When you're ready to visualize all teams again - and any conflicts between them - open the Teams menu and select All teams.

Create a New Issue

To create a new issue and add it to the board, hover over the bottom of a column and click the sign.

Create a new issue

This will open the Jira Create Issue dialogue. Once created, the issue will be added to the selected location on the board.

Jira create issue dialogue