String.prototype.startsWith is case sensitive. This may be what's confusing you. You may want to convert to a common case before comparing:
fValues.filter(row => row.picklistName.toLowerCase().startsWith(searchValue.toLowerCase()));
You want to use the Lightning Notifications Library. This has the capability to show toasts or notifications.
You can reference the documentation and examples here
Here is a basic example shown from documenation:
<lightning:button name="notice" ...
Neither of lightning:select or lightning:combobox support multiple select (as per docs). Lightning:dualListBox is the appropriate component for this-
this is a very powerful and tricky thing to master in JS.
In your code this holds the context which calls different methods in the aura component framework.
Using this in different loops changes the context values which it stores. So , while calling another helper method outside of a loop or callback function may or may not work at all times.
To overcome ...