I am using a triggered email to send email to a person who registers to my page. The triggered email works fine. However, I would like to send a next email to persons who open the previous triggered email. I don't know how to do that. I usually do that in a Journey, but not with triggered email. I can't find any feature in Journey Builder that relates to triggered email. If there is anyway to achieve my gold, please give me a suggestion.
You can also use the API entry event, and keep all emails in Journey Builder
My suggestion will be to use the Automation Studio Audience entry event. The automation will contain a Query activity, looking into Open data view: https://help.marketingcloud.com/en/documentation/automation_studio/using_automation_studio_activities/using_the_query_activity/data_views/data_view_open/
You can then select the open events for that specific email, and use the Subscriber Key to inject the recipient into the journey.