how can i make only certain customers able to order a product with log in, to order their products which are custom made only for that certain customer. and that these products are not seen for others but only for customer who ordered custom product.
Hi @custom2 for a small set of products & customers that would be a content-gate customization, aka wholesale customization or membership customization. Probably using metafields instead of tags to clearly associate products in the admin.
For lots of products & customer combinations you may need to use an app like locksmith.
To DIY the theme customization there are plenty of exiting duplicate conversations on the topic where roughly theme customizations wrap parts of theme code with logic that checks some condition:
If you need this customization then contact me directly by mail for services. Please always provide context, examples: store url, theme name, post url(s) , or any further detail. Contact Info in signature.