Salesforce Integration

• In which components of Salesforce are global variables used?
• What are the most commonly global variables in Salesforce?
• What are Formulas in Salesforce?
• What are the pros and cons of using either SAP C4C or Salesforce CRM?
• If the app that we would like to connect to Salesforce does not have an open API, is it possible to still have an integration?
• How can we display Images stored in Documents object on VF page in Salesforce?
• I have a custom application in Salesforce classic. Will I need to build a component from scratch to move it to Lightning? If we have a Visualforce page, how easy is it to convert to lightning component?
• What are the differences between Files and Attachments in Salesforce?