Recommendations for CSS and JavaScript Management in WordPress

Hi everyone,

I'm new to WordPress and I have a couple of questions that I hope you can help with:

1. Plugin Recommendations: I'm looking for free plugins that can help me manage additional CSS over the existing style and also allow me to add custom JavaScript scripts. Any suggestions on plugins that work well for these purposes?
2. Custom CSS and JavaScript: I’m learning to code and I’d like to know where to add custom CSS and JavaScript within the existing files in the WordPress app. I've used plugins so far because I wasn't confident, but I want to ensure that my styles apply universally and that my JavaScript scripts can be called everywhere. Where should I add these customizations in the WordPress files?