How do I start a workflow when a contact gets a tag, and can a workflow have several triggers?

Create the workflow in an agent and choose the Tag added trigger. Enter the tag name, and when a contact gets that tag the workflow runs. You can add as many tags as you want to one trigger, and you can use multiple triggers in one workflow, for example a tag trigger plus an On a schedule trigger. Connect them all into the workflow.

In the workflow you can use the usual steps, such as sending emails, waiting, branching on a reply, sending a notification and changing the contact's pipeline stage.

You can also ask the AI to build the workflow, for example: a 14 day database reactivation email sequence that starts when the tag reactivation is added. The AI builds the steps (send the email, wait for a reply, if they reply mark them reactivated and notify you, if not wait a few days and send the next one). A workflow built by the AI starts disabled, so turn the trigger on before it will run. After it is built you can change details yourself, such as the days between emails, and you can copy a workflow to reuse it.