Below is a code snipped to demonstrate how to insert a new record into a SharePoint list that contains complex data types like a Person field or a lookup to another list.
The Set function in line 1 is to store the ID of the newly created item in SharePoint. You can then use this ID in any further patch requests.
Lines 6 and 11, the Sample and Location field is a Lookup to another list. You always have to use that exact object structure with the odata type, Id and value.
At line 16, the Owner field is a Person column in SharePoint, once again this is how you will have to update a person column using that exact structure.
Line 25, the ReasonCode field is a multi select column in SharePoint. If you have a listbox in Power Apps that allow you to select multiple items, this is how you would add each of the selected items to the SharePoint column.