Explanation
To capture the expiration date of warranty based on installation date and warranty length (1 year by default), the administrator should use a formula as the default value of Warranty Expiration Date field on Installation object. The formula can calculate one year from installation date using DATE function or DATEVALUE function. For example, DATE(YEAR(Installation_Date__c) + 1 , MONTH(Installation_Date__c) , DAY(Installation_Date__c)) will return one year from installation date. Creating a formula field, adding a validation rule, or including Warranty Expiration Date field on mobile page layout will not capture expiration date based on installation date and warranty length. References:
https://help.salesforce.com/s/articleView?id=sf.formula_using_date_datetime.htm&type=5

Explanation
To prevent users from selecting more than one value on a picklist field, the administrator needs to change the field type from a multi-select picklist to a regular picklist, and select the "Choose only one value" checkbox on the field definition page. This will ensure that only one value can be selected on the record page and in reports and filters. References: https://help.salesforce.com/s/articleView?id=sf.customize_picklists.htm&type=5

Explanation
Compact layouts determine which fields appear in the highlights panel on record pages and in the Salesforce mobile app. To create a custom compact layout, go to the Account object manager and select Compact Layouts from the sidebar menu. Then click New and add the desired fields to the layout. References:
https://help.salesforce.com/s/articleView?id=sf.compact_layouts_create.htm&type=5

Explanation
Validation rules allow you to enforce data quality by preventing users from saving records that do not meet certain criteria.
References: https://help.salesforce.com/s/articleView?id=sf.validation_rules.htm&type=5