Can someone help me with this, please?
This is PageFly - Free Landing Page Builder. I would love to provide my recommendations for your store based on 6 years of providing solutions for about 100.000 active Shopify merchants.
Regards your concern.
That will be a bit complicated for a user to convert the whole HTML website to a Shopify theme because it is different how the code work and how the folder of the theme. So I suggest you convert a page one by one. Shopify has a function to paste the HTML code to the regular page here:
If not you can use another 3rd app page builder, such as PageFly, with drag n drop UI it is easy to use for every one, besides of that we have video for tutorials and 24/7 support will help you if you have trouble with it.
If you feel my answer is helpful, like it or mark it as a solution. Let me know if you have any questions.
Best regards,
PageFly