Responsive web designing is a concept that mandates that every website be designed to look and feel great regardless of what device you access it from, I'm sure we've all seen some websites that we can only properly access on a computer, in the "bad" old days designers didn't have to worry about mobile devices, designing websites used to be a lot easier for software engineers though I must admit that we have fancier tools than they did so in a way developers back then had it rough.
These days, the average junior web designer will admit that responsive web designing is rather tedious, some school of thought dictates that websites should be designed with "mobile-first" capabilities, what this means is that the website should be designed to work well on mobile devices before it is designed to work on larger screens like your desktops and smart TVs. This is because most traffic to a website is from mobile devices.
However, mobile-first web designing is rather tedious because programmers make use of a computer when designing websites as such it's a lot easier to design to the screen size that you're programming with. In principle mobile-first designing is better, in reality, it's just easier to do desktop-first. This is why most websites look better on PC than on mobile (for now).
A lot of strides are being made to make responsive designing a lot easier, bootstrap does a lot of heavy-lifting for web developers nowadays. However, for junior developers, bootstrapping can easily become a nightmare if you make a mistake after using it, I believe it is best to properly understand CSS before making use of bootstrap, this way you're not just copying code, you're borrowing code that you understand and can manipulate to fit your specific needs.