Each material should be recorded in it's own record not in different fields fo the same record. In manufacturing, this related table of materials is called a "Bill of Materials" or BOM.
Products::__pkProductID = BOM::_fkProductID
Materials::__pkMaterialID = BOM::_fkMaterialID
You can place a portal to BOM on the Products layout to list and select a Materials record for each given Products record. Fields from Materials can be included in the Portal to show additional info about each selected Materials record and the _fkMaterialID field can be set up with a value list for selecting Materials records by their ID field.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
Yes, this is really simple.
Go to "File" - "Manage" - "Database"
And go to the Relationships tab.
Select your table and hit the icon with the two green arrows on the bottom to duplicate the Table Occurrence.
Now drag a line from the Category field in the one Table Occurrence to the Category field in the other Table Occurrence.
You have created a relationship that says that records will be shown where the Category in table occurrence A equals the category in table occurrence B.
Now just create a portal on your layout that shows records from table occurrence B and you are all set.