How do I sum the values for rows with specific string matches

You can use dplyr functions, group_by() and summarise().

You can learn more about data wrangling in general in this free ebook

Or, if you need more specific help, please provide a proper REPRoducible EXample (reprex) illustrating your issue.

Also, please be aware that this is not supposed to be a support ticket where we follow along with all the stages of your project, this is mostly a Q & A forum so you should only ask one reasonably well defined question per topic, but you can open as many topics as you need.