One note on terminology: "CH3" is a methyl group, and not methane. Carbon makes 4 bonds, so CH3 is not a stand alone molecule, but rather a substituent/group that must be bonded to another carbon. Methyl is an alkyl group, and such groups end in "-yl". Alkanes are stable molecules where all the carbons make four bonds, and their names and in -ane. For example CH4 is methane.
Also, "methyl" doesn't really mean just one carbon. It literally means CH3. CH2 and CH have other names, but its probably best not to get too bogged down in terminology and focus on the concepts also.
(CH3)3C is also not a complete molecule, because the central carbon is only making three bonds. It is called a "tert-butyl" group. This article about isomers and terminology might be helpful: https://www.masterorganicchemistry.com/2011/11/10/dont-be-futyl-learn-the-butyls/