I am not sure if I understand the question. Do you want to show only the items in a grid, where the associated objects match your selection?
If so, you could just add the dropdowns as search fields.
If you want to set the relation to A or B for all Items in the List, you need to do it item by item, or use a microflow that gets the selection from a helper object (dropdown) and does a batch processing.
I'm not completely sure what you are asking. But if you want to change values in a list, you can use a list view and set the setting "editable" to yes. From your model, it looks like you you have three tables, with one table in between A and B that has many to 1 relationships. I would make this middle table the data source for your list view, and then you can reference the other two tables with reference selectors.
here is the documentation on listviews.
Alternatively, you can use a datagrid. I would make the data source the middle table, and then you can add columns and map them to the associated tables. Then you can double click the column and set it to be editable, and therefore allowing you to edit the values.