Welcome on the ODK Community forum @kateanelson !
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You can achieve what you want to do with a appropriate choice_filter clause.
Here is an example :
Choice_filter_based_on_select_one_questions.xlsx (9,3 Ko)
The calculate (line 9) concatenates the products names selected in previous select_one questions.
In the choice_filter of the select_multiple question you just need to check whether the "used_products" variable contains the current option name.