Given a document
That doesn't specify a font the font will be as specified by the document class, or if that doesn't specify a font family then the LaTeX default.
In this case
article specifies a default size of 10pt but does not specify a font family so the format default is used.
In pdflatex or latex that is encoding OT1 the original 7-bit TeX encoding and family cmr (Computer Modern Roman). If you process the document the end of the log shows
showing that the only font used is the Type1 (PostScript) version of cmr10.
If you process the same with lualatex then you get the format default encoding TU (Unicode) and format default of
lmr (Latin Modern). The end of the log again shows the full path to each font used (just one on this case)
Showing the OpenType version of Latin Modern Roman 10
These OT1 (or TU) defaults are set in the format, specifically the file
base/fonttext.ltx which has the lines
Where the true branch is executed in pdflatex and latex and the false branch is executed in lualatex and xelatex.
In principle you could copy
fonttext.cfg modify it to use some different font set and then rebuild the format, the
.cfg file if it exists will be used in preference to the
.ltx file. But really you should never do this, it has no advantages over simply using the same commands to change the font in the document or in a custom document class, and it makes your "latex" format incompatible with every other latex installation. It sort of made sense when we added this in the early 1990s before widely available common distributions such as miktex and texlive were available, and different sites had different fonts available (and changing fonts in-document could take minutes rather than fractions of a second) but now it should be regarded as a obsolete feature that is never used.