The best way to configure a thank you email to be sent after the form on the "Request a Demo" landing page is submitted is to configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted. An autoresponder email is an email that is automatically sent to a prospect after they complete a desired activity, such as submitting a form or clicking a custom link. You can use autoresponder emails to thank your prospects, confirm their actions, or provide them with additional information or content. A completion action is an action that Marketing Cloud Account Engagement executes after a prospect successfully completes a desired activity, such as submitting a form or clicking a custom link. You can use completion actions to perform actions on the prospects who complete the activity, such as adding them to a list, assigning them to a user, or changing their field values. To configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted, you need to go to Marketing > Forms > Forms > Request a Demo > Completion Actions > Add New Action > Send Autoresponder Email, and select the email that you want to send.

To view a prospect's Marketing Cloud Account Engagement landing page submission activity on the syncing contact in Salesforce, an Administrator can use the following features:
The Marketing Cloud Account Engagement Activities Visualforce page, which shows a detailed history of the prospect's interactions with Marketing Cloud Account Engagement assets, including landing pages, forms, emails, and custom redirects.
The Salesforce Activities section, which shows the tasks and events that are logged for the contact, including Marketing Cloud Account Engagement landing page submissions. Reference: [Marketing Cloud Account Engagement Activities Visualforce Page], [Salesforce Activities]

Explanation
An autoresponder is a type of email that is automatically sent to a prospect when they perform a specific action, such as filling out a form, downloading a file, or registering for an event. To send an autoresponder, a marketer must create an email template using the classic email builder in Marketing Cloud Account Engagement. An email template is a reusable email design that can be customized with dynamic content and personalization. A test email, a one-to-one email, and a list email are not suitable for sending an autoresponder, as they are either not reusable, not automated, or not personalized. References: : Create an Autoresponder Email

The best way to track the number of visitors who access a file on a web page that the company does not own on the company website is to use custom redirects . Custom redirects are used to track links such as banner ad clicks, links to third-party sites, links on social media, and access to files hosted outside of Account Engagement. Link clicks appear as activity on a prospect's record. Page actions (A) are not a valid option, as they are used to track and automate actions based on a prospect's page views, not link clicks. Account Engagement form (B) is not a valid option, as it is used to capture and update prospect information, not track link clicks. Account Engagement tracking code (D) is not a valid option, as it is used to track and cookie visitors on the company website, not on a third-party site. Reference: Custom Redirects

The limitations of connecting Marketing Cloud Account Engagement to a Salesforce Sandbox account are: if connected to a sandbox first, when you create your production connector, all prospects will not automatically queue to sync, and Marketing Cloud Account Engagement can't "wipe" data, so you will need to manually delete any prospects and data that is brought into your Marketing Cloud Account Engagement account from the sandbox connector before enabling the connector for your production instance. A Salesforce Sandbox account is a copy of your production environment that you can use for testing and development purposes. Marketing Cloud Account Engagement Sandboxes are test versions of a Marketing Cloud Account Engagement Business Unit that you can provision within a Salesforce Sandbox. However, Marketing Cloud Account Engagement Sandboxes have some limitations that you need to be aware of before using them. One limitation is that if you connect Marketing Cloud Account Engagement to a Salesforce Sandbox first, and then create a production connector, the prospects in your Marketing Cloud Account Engagement account will not automatically sync with the production environment. You will need to manually sync them or use an automation tool to do so. Another limitation is that Marketing Cloud Account Engagement cannot erase the data that is brought into your Marketing Cloud Account Engagement account from the sandbox connector, such as prospects, fields, and assets. You will need to manually delete them before enabling the connector for your production instance, or else you will end up with duplicate or unwanted data in your Marketing Cloud Account Engagement account1112 Reference: 11: Marketing Cloud Account Engagement Sandboxes: What They Can, and Can't Do12: Use Marketing Cloud Account Engagement Sandboxes