Flow Solutions are pre-built flows or templates that can be installed from AppExchange and customized to meet specific business needs. For example, Flow Solutions can provide common use cases such as lead generation, document generation, payment processing, and more. In this case, the administrator can use a Flow Solution that provides a mortgage calculator and generates leads for loans. Flow Solutions are easy to install and configure, and can help save time and budget for projects. References:
https://help.salesforce.com/s/articleView?id=sf.flow_solutions.htm&type=5

The likely cause for why a rehired user is no longer able to access records they worked on before leaving the company is that the records were manually shared with the user. Manual sharing allows granting access to individual records to specific users or groups. However, manual sharing is removed when a record owner changes or when a user's role changes. When a user is deactivated, their role is removed and any manual sharing involving that user is deleted. When a user is reactivated, their role is restored but manual sharing is not. Therefore, the rehired user will not have access to records that were manually shared with them before deactivation. The stage of Opportunity records, permission sets, or record type of Opportunity records are not likely causes for why a rehired user is no longer able to access records they worked on before leaving the company. References: https://help.salesforce.com/s/articleView?id=sf.sharing_manual.htm&type=5

Component visibility allows you to restrict the visibility of a related list based on a permission set.
References:
https://help.salesforce.com/s/articleView?id=sf.dynamic_forms_component_visibility.htm&type=5

Record types and sales processes allow you to have different page layouts, fields, required fields, and picklist values for different types of opportunities.
References: https://www.salesforceben.com/salesforce-record-types/
https://trailhead.salesforce.com/content/learn/projects/create-an-opportunity-record-type-for-npsp/create-and-ma

Formula fields are types of fields that calculate a value based on an expression or formula that references other fields or constants. Formula fields prevent administrators from changing their field type once they are created because they may be referenced by other functionalities such as reports, validation rules, workflow rules, etc., that depend on their data type and value. If a formula field is referenced by other functionalities, then changing its field type may cause errors or unexpected results. References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_formulas.htm&type=5