Spread Out Conditional Field Record For Free

You cannot make field read-only based on picklist value, however you can create a validation rule and restrict text field to change when you have particular picklist value. Something like below. Else you would need to build custom page to make field read only using custom logic code.

Salesforce Classic: Click on Setup | Manage Users | Permission Sets. Open the Permission Set that has the 'Edit Read-Only Fields' added in it. Go to System permission. Deselect the option 'Edit Read Only Fields' Save.

Conditional Visibility allows you to specify when you want a particular page component to be visible. We can use this to customise our pages to make information appear when it is relevant to the business process, and be hidden when not, so that we can keep the page as simple and concise for our users as possible.

Use Visualforce Pages or Lightning Web Components to show or hide your components based on logic and user permissions. If you are a Salesforce administrator, you can also control the visibility of your fields with permission sets or objects based on user profiles.

You can set the field as Read Only on page layout. From the "Quick find" box on the left search for "Field Accessibility" then choose your "obj1". Select "View by fields" and choose "field1". For each profile select "Read Only" besides for "Billing User".

You can do this by assigning a different record type with a workflow. This record type would have a separate pagelayout with the fields you require to be read only set as such. You may have to explore VisualForce pages. You can embed one in the Opportunity layout and have it act upon your fields.

Setup ---> Customize ---> Objects ---> Fields ---> Click on the field you want to make read-only ---> Click on button 'Set field-level security' ---> here make the field read only for the profiles you want. note: Formula fields are always calculated, and therefore always read-only. Hope this helps you!