@Kopistars11 - Sounds like you're talking about creating a one-to-many relationship between a person (one) and (many) companies.
Typically, this is done by creating different tables and relating them together using a common identifier.
However, based purely on what you have described, the simplest method is to run everything off a single list.
1. Create a new SharePoint List with a People Picker field and a Single Text Field called Company, as well as another Single Line Text field called Responsibility (or whatever name you want).
2. Add an Edit Form control into your App and associate the DataSource property to this List
3. Insert a Dropdown or ComboBox control into the Company DataCard.
4. In the Items property of that control, enter your Companies. This can be from a separate list, but for simplicity, let's add the following into the Items property:
["Company A", "Company B", "Company C"]
5. Delete the Text Input field for the Company DataCard
6. Address the Y and X DataCard property errors by replacing the deleted fields with the name of your Dropdown/ComboBox control. For the Update property, replace it the formula with:
'Your ComboBox Control'.Selected.Value
7. Add a Button control into your app and in the OnSelect property, enter:
SubmitForm('Your Form')
8. Add another Button control into your app and in the OnSelect property, enter:
NewForm('Your Form')
9. Add a Gallery control into the same screen and connect it to the same data source.
10. On the OnSelect property of the Gallery control, enter:
EditForm('Your Form')
11. On the Item property of the Form, enter:
'Your Gallery Name'.Selected
12. In the Default property of the Dropdown or DefaultSelectedItems property of the ComboBox control, enter:
Parent.Default
13. On the Items property of the Gallery, enter:
Filter(
'Your SharePoint List',
'Your People Field'.Email = User().Email
)