bootstrap-npm-starter | Starter template for new building | Theme library

I'm not a super expert and not sure if the question is clear enough, but I would not recommend concatenating Libraries like Boostrap into a single main file in together other JS files outside the framework, due to performance issues and possible crashes in between libs due to definitions, and the possibility to update without a build process and also your site might be penalized by google engine.

Besides that, Boostrap normally already provides .min.css and .min.js already compressed/minified/uglified ready to use if you are not gonna change anything from the original design patterns, avoid using uncompress files if you are not gonna customize it.

for the rest in regards to other custom libraries or vanilla's JS create by you, you can use grunt-contrib-concat also if you wanna check the performance you can use PageSpeed Insights with the result will know what exactly needs to be applied to get better performance and optimization.