VS 2019 Create a List for multiple entries

I need to create a List to store multiple values like following example:

I need to loop through all filenames in a folder.
I compare the filename against keywords. When the keyword exists in the filename we want to store
both the keyword and the filename to a List, for example:

"this is a beautiful filename.doc" <-> keyword "beautiful"
we store filename and keyword beautiful

"this is also a beautiful next filename.doc" <-> keyword "beautiful" and keyword "next"
we store filename and keyword beautiful and next

I am kind of lost because later on I want to return for example all filenames that match the keyword "beautiful"