Incorrect Russian character in xelatex CMU Concrete

You didn’t actually load CMU Concrete. This MWE works fine in TeX Live 2020:

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\geometry{tmargin=2cm,bmargin=2cm,lmargin=2cm,rmargin=2cm}
\usepackage[russian]{babel}

\tracinglostchars=2
\babelfont{rm}{CMU Concrete}

\begin{document}
Б\'{о}льший №
\end{document}

One advantage \babelfont has over the other approaches is that babel will load the font with Script=Cyrillic and Language=Russian, and switch to them whenever you select the Russian language. On LuaLaTeX, you could even switch automatically whenever you type in Cyrillic, with no other commands.

Cyrillic is enough like Latin that you can often get by with the default settings, but some fonts will give you Bulgarian letters instead of Russian unless you select the language as an OpenType font feature.

Use a font, that properly supports the languages, you are using. Here is a solution with Noto:

\documentclass[12pt]{article}
\usepackage{noto-serif}
\usepackage{noto-sans}
\usepackage{noto-mono}
\usepackage[russian]{babel}
\begin{document}

  \textrm{Б\'{о}льший №}
  \textsf{Б\'{о}льший №}
  \texttt{Б\'{о}льший №}

\end{document}

I think that being in XeLaTeX and the characters are in unicode...it is necessary to use,

\usepackage{fontspec}
\setmainfont{Arial} %<---- a random set of characters.

---

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage[russian]{babel}
\usepackage{fontspec}
\setmainfont{Arial}
\geometry{tmargin=2cm,bmargin=2cm,lmargin=2cm,rmargin=2cm}

\begin{document}
Б\'{о}льший №
\end{document}

Using Noto Serif,

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage[russian]{babel}
\usepackage{fontspec}
\setmainfont{Noto Serif}
\geometry{tmargin=2cm,bmargin=2cm,lmargin=2cm,rmargin=2cm}

\begin{document}
Б\'{о}льший №
\end{document}