fontspec / babelfont: Mixing Chinese and English | Latin Font Different in Chinese vs. English Text | Google Noto Fonts

You could try the (new) multiscript option. But you will get a few warning from fontspec:

\documentclass{article}


\usepackage[english]{babel}
% babelprovide for *english* (default language of the document)
\babelprovide[
    main,
    import,
    language = Default]
    {english}

% babelprovide for *chinese-simplified*
\babelprovide[
    import,
    language = Chinese Simplified]
    {chinese-simplified}
\directlua{
luaotfload.add_multiscript
 ("latn-hani",
  {
    Hani = "Noto Serif CJK SC:mode=node;script=hani;",
  }
 )
}
\babelfont{rm}[RawFeature={multiscript=latn-hani}]{Noto Serif}


\begin{document}

\selectlanguage{english}ABCabc 你好
\selectlanguage{chinese-simplified}ABCabc 你好

\end{document}

It is indeed the case that the Latin glyphs from "Noto Serif CJK SC" is notably lighter than the corresponding weights from "Noto Serif". I am not aware of any documents explaining this design choice. However, it is worth noting that the Latin glyphs from "Noto Serif" would look like a (slight) emphasis within a span of Chinese text. One can see this in Ulrike's answer. An alternative is to explicitly mark the Latin characters with a language:

ABCabc

{\selectlanguage{chinese-simplified} 简化字总表 ABCabc 简化字总表}

{\selectlanguage{chinese-simplified} 简化字总表 \foreignlanguage{english}{ABCabc} 简化字总表}

ABCabc

In the second line, the Latin text blends into the Chinese text while it stands out in the third line.

Now Noto comes win many different weights. If CJK-Regular is lighter than Latin-Regular, we can try to combine CJK-Regular with Latin-Light:

\documentclass{article}

\usepackage[english]{babel}
% babelprovide for *english* (default language of the document)
\babelprovide[
    main,
    import,
    language = Default]
    {english}

% babelprovide for *chinese-simplified*
\babelprovide[
    import,
    language = Chinese Simplified]
    {chinese-simplified}

%% Choose actual fonts for different font variants.
\babelfont{rm}[
  UprightFont=*-Light,
  BoldFont=*-Medium,
]{Noto Serif}
\babelfont[chinese-simplified]{rm}{Noto Serif CJK SC}

\begin{document}

ABCabc

{\selectlanguage{chinese-simplified} 简化字总表 ABCabc 简化字总表}

{\selectlanguage{chinese-simplified} 简化字总表 \foreignlanguage{english}{ABCabc} 简化字总表}

ABCabc

\end{document}

The match is not perfect, but quite close. In principle it would also be possible to choose a slightly darker CJK. I do not have those installed right now, though.

To expand on the latin-in-chinese topic:

Two parts:

The first part is that Babel does not change the font according to which Unicode block the glyphs are in, like package ucharclasses does. Addendum: But see the comment about recent Babel.

The second part is that Noto Serif CJK SC contains some non-CJK glyphs as well.

Noto Serif has glyphs for Latin, Cyrillic, Greek, and a few other things.

Noto Serif CJK SC has the main parts of Latin, Cyrillic and Greek, as well as hiragana, katakana, hangul, bopomofo, and of course a large part of CJK.

ucharclasses:

MWE

\documentclass[12pt]{article}
\usepackage{xcolor}
\pagecolor{red!3}
\usepackage{fontspec}
\newfontface\fcjk{Noto Serif CJK SC}
\newfontfamily\feng{Noto Serif}

\usepackage[CJK]{ucharclasses}
\setTransitionsForCJK{\fcjk}{\feng}

\begin{document}
\large
{\color{blue}Noto Serif}

\feng 
\begin{tabular}{rl}
Latin & ABC abc \\
Currency symbols & ₣₤₥₦₧₨ \\
Cyrillic  & ЄДФЦжцабвг \\
Greek & ΣΤΥΦζεδγβα  \\
IPA \& Phonetic & ɐɠɰʁ ᴟᴞᴣᴇᴈᴗ \\
Letterlike Symbols & ℜℏ℈℃ℓℒ™ℵ⅀ \\
Modifier Tones & ꜀꜂꜈꜉꜊꜋꜌ꜗꜘꜙ \\
etc & ... \\
\end{tabular} 
\bigskip

\rmfamily
{\color{blue}Noto Serif CJK SC}

\fcjk
\begin{tabular}{rl}
Bopomofo & ㄆㄇㄈㄉㄊ \\
Box Drawing & ┢┤┩╆ \\
CJK & 㑣㐦㐧㕔 \\
some Latin & ABC abc \\
some Cyrillic  & ЄДФЦжцабвг \\
some Greek & ΣΤΥΦζεδγβα  \\
Hangul &  ᅣᅦᄄᆻ \\
Hiragana & あいいうえおかが \\
Katakana & アイウエオカガ \\
Letterlike Symbols & ℜℏ℈℃ℓℒ™ℵ⅀ \\
etc & ... \\
\end{tabular} 
\bigskip

\feng
\textit{ucharclasses}: {\small Changing fonts without having to insert font-changing code} -- Latin text. Chinese text: 紫薇北斗星  Modern Greek: Διαμ πριμα εσθ ατ, κυο πχιλωσοπηια Ancient Greek: Μῆνιν ἄειδε, θεά, Πηληϊάδεω Ἀχιλῆος. And regular latin text.

\end{document}