请问that is是什么成分？和后面的句子是如何联系的？

They believe what is behind this is the deep-rooted drawback of Chinese cultural products, that is, intermediate products are non-terminal, low-end products cannot be made independently, and processed goods are non-branded.请问老师，that is是插入语吗？若是的话，那这句话应该不对吧？intermediate products和后面几项都没主谓成分了？

They believe what is behind this is the deep-rooted drawback of Chinese cultural products, that is, intermediate products are non-terminal, low-end products cannot be made independently, and processed goods are non-branded.

that is 是插入语，表示后面的三个并列分句是对前面划线部分的讲解、说明。