If you already possess an online store on Shopify, it has a myshopify.com URL by default. If you want to change the web address that's displayed to customers to the domain name that you have registered on Dynadot, then you need to add it manually following the steps outlined in the help file link below.
How do I set up my domain with Shopify? https://www.dynadot.com/community/help/question/shopify-domain-set-up
Setting up your domain with Shopify helps you establish your brand and build trust in your online store. However, please rest assured, it is not mandatory at all to link your domain to your Shopify store unless you want to. Nothing will happen that will negatively impact either your domain or your online Shopify store if you don’t do it, they just won’t be linked in the way outlined above.
We hope this answers your query. However, if there is anything else that has been left unclear, please do not hesitate to contact us.