Hello.

Layouts are stored in database https://abantecart.atlassian.net/wiki/spaces/AD/pages/9732104/Manage+Product+Layout
Blocks usually in theme folder like extensions/theme_name/storefront/view/theme_name/template/ directory

Hello

The purchases may come later to GA (about one day) but you can use google tool https://tagassistant.google.com/ to see what is sent after you complete the order

Hello.
If your theme is not BS5 it may require update

Hello.
There is no such extension available, but the AbanteCart marketplace is built on the AbanteCart core. If you are looking for a similar paid solution, please contact managed@abantecart.com

Hello.
You have two options to add code to your site: either use the HTML block feature or install an extension from a third party.
https://abantecart.atlassian.net/wiki/spaces/AD/pages/12419073/HTML+Block
https://marketplace.abantecart.com/age_verification

Hello.
When you click Buy Now you need to go through the checkout process (enter delivery address or login) to see the Shipping methods selection.

Hello.
Change additional icons in storefront/view/default/image/ https://github.com/abantecart/abantecart-src/blob/1.3.5/public_html/storefront/view/default/template/common/head.tpl#L35

Hello.
Right click on the admin logo and click Open in a new window. You will see where logo is located
https://demo.abantecart.com/admin/view/default/image/logo.png