A concrete wall stops wifi pretty well.

How thick a concrete wall is needed to stop wifi? Or does the concrete wall contain fine wire mesh?

If by "fine" you mean minimum ~10mm rebar then yes

Have you never walked into a building an lost cell reception?

Found a table with attenuation by frequency and building material, e.g. low-emission glass can block more than concrete, https://www.signalbooster.com/blogs/news/how-much-which-buil...

                        800Mhz     1900Mhz
  1/2" Drywall          2.03 dB    2.43 dB
  Venetian Plaster      7.91 dB    16.22 dB
  6" Concrete Wall      10.11 dB   19.41 dB
  Glass Window          4.35 dB    4.38 dB
  Low Emission Glass    33.8 dB    33.8 dB
  Brick                 7.57 dB    14.66 dB
  Solid Wooden Door     6.11 dB    12.33 dB