allbidder

{
        test: /\.((c|sa|sc)ss)$/i,
        use: [
          'style-loader',
          {loader: 'css-loader', options: {sourceMap: true, importLoaders: 1, modules: {
            localIdentName: '[name]__[local]--[hash:base64:5]',
          }}},
          {loader: 'postcss-loader', options: {sourceMap: true}},
          {loader: 'sass-loader', options: {sourceMap: true}},
        ],
      },

для "production" 'style-loader' заменить на MiniCssExtractPlugin.loader и sourceMap: false

import { HashRouter } from 'react-router-dom'

<HashRouter>
  <App />
</HashRouter>