Most of them these days are going to be using at minimum divs and a fluid style. I absolutely will not use tables unless there is no other alternative.
Anything I do custom these days is always done mobile first. Usually leveraging bootstrap 5.3+ or Pico.css, depending on the project. I'm approaching a point in several major projects I am working on currently (CMS, Forum and Blog, well, actually just a core that will eventually drive all of them) so I am going to have to make those same choices. At the moment I am leaning towards Bootstrap but have not decided entirely. I like Pico a lot because of its semantics and you can pair it will Bootstrap's grid system, but..... BS 5.3+ has "themes" so will most likely go with that and expand the theme manager tooling to include build tool integration. Who knows, I'm a ways from that point yet.
No matter which I go with I will most likely set it up in a master grid leveraging breakpoints so that I can provide a widget type UI without having to worry (much) about breakage. Although I will say, that I may not even go through all that trouble, and just build a React front-end for it, it is being built to use either.