Upon changing the server, I have encountered an error within the error log and have been unable to send or receive emails. The specific error message is as follows:

"2023-08-24 19:35:31 - AMail. transport: Symfony\Component\Mailer\Transport\Smtp\EsmtpTransport: Connection could not be established with host 'smtp.XXXX.com:587': stream_socket_client(): Unable to connect to smtp.dreamhost.com:587 (Connection refused)"

I have engaged with my service provider in an attempt to resolve the issue. According to their assessment, everything appears to be functioning correctly on their end.

I have also verified that email settings on the server site are properly configured and that email sending is operational. In addition, I've conducted tests using ports 465, 587, and 25, but the issue persists.

Interestingly, I am able to successfully log in to my server email account and send/receive emails directly from there. However, the problem arises when attempting to send emails through my application.

I have made an effort to find a solution to this issue by researching online resources, including the discussion on the AbanteCart forum here: forum.abantecart.com/topic/4200. Unfortunately, I have not been able to pinpoint the exact cause of this problem.

Given your expertise in this field, I kindly request your assistance in diagnosing and resolving this matter. Your insights and guidance would be greatly appreciated.

Thank you for your time and consideration. Please let me know if you require any further information from my end.

I'd like to know in detail, too.

I have installed AbanteCart with the BootStrap Theme. I have realized that there is a space below the footer block. The footer Block doesn't fill the entire footer. How do I remove this space?

You have pretty well understood it.  Be sure your two sites are NOT on the same machine and have different IP addresses.
the URL is the SAME for both sights -the difference is in the DNS (nameserver) which is controlled within your domain nameservers info at your registrar.

And you have picked up on the syncing issue.   As to what "big companies" do,  there are several solutions.  from simple to complex.

What kind of volume to you have? That will help you decide how much effort and cost you need to insure your info is relatively in sync.
Various solutions -  you can move your DB to one different location and let both app locations write to the same DB -  then make more frequent backups of the DB  in a location other than your site location.   eg.  if you have Softaculous you can set your backup for just the DB  and save the backup on a different location like a dropbox account or AWS if you use that.

Look into what your backup options are from your hosting company -  are you having a daily backup with several generations auto ran  and stored off your site server??   
Your hosting company should able to provide you more specific options that work within their framework

Do you get notification when your site went offline?

There is also the ability to have a hot-swap raid backup  on some hosts -  that does cost more,  but what it is two drives assigned to your account -  and they both get written to almost simultaneously,  if the main one goes down,  the hot swap is actually pulled physically and placed in a new location -  most host who provide this monitor your site and if there is a mechanically failure of a drive they do the swap  quickly.

Hello.
Try to compare the files with the source, maybe something was modified https://github.com/abantecart/abantecart-src