The words "you" and "You" refer to the Hebrew believers to whom the author wrote. The word "they" refers to people of Israel after Moses led them out of Egypt. The first quotation comes from the writings of Moses. God reveals in this passage in Hebrews that Moses said he shook at seeing the mountain.
The author gives a contrast between what believers in Moses' time had while living under the law and what present day believers have after coming to Jesus under the new covenant. He illustrates the experience of the Israelites by describing how God appeared to them at Mount Sinai.
The implicit information can be stated explicitly. Alternate translation: "For you have not come, as the people of Israel came, to a mountain that can be touched" (See: [[rc://en/ta/man/translate/figs-explicit]])
This means that believers in Christ have not come to a physical mountain like Mount Sinai that a person can touch or see. This can be stated in active form. Alternate translation: "that a person can touch" or "that people can perceive with their senses" (See: [[rc://en/ta/man/translate/figs-activepassive]])