The latest Metronic comes with the new Bootstrap 5.3.0 update.
To add your own scss file you just need to add it under your own folder src/sass/custom/_my-styles.scss and include it in the /src/sass/style.scss as shown below:
// Custom @import "custom/my-styles";
Then recompile your assets folder with Gulp or Webpack.
Please note that the build tools are required only in the development environment just to compile the assets when the source folder files are modified. In the hosting/server deployment, you will only need the compile assets, no need to install the build tools dependencies there.