..."virgin" TeX...knows just primitive commands, no macros. Plain TeX is the set of macros (developed by Knuth) which makes TeX usable in everyday life of a typist. ... The available commands can be classified into primitive commands and macros. ... The "virgin" TeX knows only the primitive commands. ... Formats (plain TeX, LaTeX, etc.) extend TeX's vocabulary by defining macros. ...For example, plain TeX defines macros \item, \rm, \newdimen, \loop, etc. Plain TeX defines about 600 macros.
Yes of course; see this answer I wrote about typesetting with “virgin” TeX: https://tex.stackexchange.com/a/388360/48 (it's not easy). “Virgin” TeX is never (and was never) used by typical users, and is used only by the system administrator (or these days, the people behind the TeX distributions) to pre-load formats (like plain or LaTeX).
Knuth wrote both the TeX program and the “plain” set of macros; when you start `tex` it is with `plain` that it starts up, and The TeXbook describes both the TeX program and the plain format without being careful to distinguish what comes from where (you have to look at Appendix B to see the proper definition of plain.tex), so when we speak of TeX as Knuth intended/imagined it to be used, it is plain TeX that is meant.
..."virgin" TeX...knows just primitive commands, no macros. Plain TeX is the set of macros (developed by Knuth) which makes TeX usable in everyday life of a typist. ... The available commands can be classified into primitive commands and macros. ... The "virgin" TeX knows only the primitive commands. ... Formats (plain TeX, LaTeX, etc.) extend TeX's vocabulary by defining macros. ...For example, plain TeX defines macros \item, \rm, \newdimen, \loop, etc. Plain TeX defines about 600 macros.
https://tex.stackexchange.com/questions/97520/what-is-plain-...