Hi @Veronica22 ,
This is tricky but not impossible.
Below are some scenarios since I'm not sure what you want at the end,
Scenario 1.You can use 1 Form to perform this both actions.
Let's say when you click 'New Record/Add Record' then you open a Form. using JavaScript, detect FormType and only show sections of Form where you want to add the information.
Once saved, hide/show sections or Tabs of your Form where you have 'All Details' (You can have those Read-Only).
To Edit the record, use a Toggle Field 'Edit' when checked/true fire JS to show only editing section of the Form. Once saved, revert the Flag and show 'All Details' section.
Scenario 2.
When you select and open a Record, does all fields in them are Read-Only? Then when you select Edit, you want to Open another Form to Edit?
=> If this is the case, then your main (details) form should be Read-Only and on Ribbon Button you can use Custom Edit Button when clicked, open new Form with editing action.
Hope this helps