Ok so here’s a suggestion of how I would do it:
Let’s say you have a button “Add competency” on the employee page.
This button would lead to a page with a basic dropdown menu displaying all the different competencies available (“Competencies” collection).
You select the competency wanted in the dropdown, click on a validation button that will add the competency to the user’s page (“User” collection or “employee” collection, “competencies” field).
Now if this competency hasn’t been yet created by the employer, you can display on that same page a “Create new competency” button. Clicking this button will lead to a page/modal with a basic form to create a new competency asking for example for the name of the competency and a quick description of it (“Competencies” collection).
After sending your form, you’re back to your add competency page with the competency just created displayed in your dropdown list. Select it, validate, it’s been added to your employee.
Now regarding the activities.
You could create a page displaying the details of a competency.
This is where your activies could be showing.
A button “Add activities” would lead to a page with your multi-select dropdown displaying all the activities available for this competency (“Activities” collection, filtered by current competency).
As for the competencies, if one of the activity is not yet created, on the page with your multi-select dropdown, you could have a button “Create activity” leading to a basic form to create your acitivity with fields like name and description as well (“Activities” collection).
After sending your form, you’re back to your add activities page with the acitivity just created displayed in your mutli-select dropdown list. Once the activities selected, validate* and back to your competency detail page with the activities displayed.
*Only specificity with the multi-select dropdown, it triggers the action as you’re selecting or deselecting an option. You have to set it up in the component parameters when creating it. Still displaying a validation button gives your user the idea that he does the action but your information has already been updated to your database.
Let me know if this is useful