How do you make such clean and modern landing pages?

Can't upvote this enough. Knowing how to "steal" the essence of something great and make it your own is extremely underrated. That strategy lies at the heart of all noteworthy creativity and innovation.

What you absolutely don't want to do is start with a blank canvas in an attempt to rediscover centuries of design innovation from scratch. It's an impossible task, and you're just going to produce hideous designs.

You also want to avoid unnecessarily copying all of the superficial trimmings of someone else's design. That's called cargo culting, after the tribes who thought that copying random things like building a runway would magically cause planes full of people to arrive with amazing goods and technologies. Not only does cargo culting reveal the fact that you don't actually understand the principles behind what makes something work, but it also makes it quite clear exactly who you ripped off, and is likely to upset them and harm your reputation. It's the fine line between inspiration and plagiarism. (I actually think the line is pretty think if you're not lazy.)

For example, one of the underlying principles that makes IH look decent is that bright, rainbow colors look great on a dark background. But someone copying IH superficially would use the exact pink, blue, and dark blue colors that I'm using, rather than recognizing the underlying principle and making their own superficial choices.

If you looked at the websites that inspired IH, you would never guess they inspired it, even though I completely copied their underlying principles. However, if you look at something like Nathan Latka's website, it's pretty obvious how much his designer copied superficial easy-to-identify details from IH:

Here's a tip: Never use just one source. If you want to be sure you're building on the underlying principles of a design rather than copying superficial things, then start by finding multiple sources that have a similar vibe. The things they have in common are likely to be the underlying principles, and the differences are likely to be the superficial choices.

Also, this works for literally everything creative, not just web design. It's how I came up with the business idea for IH itself. It's a cross between the underlying principles between Nomad List and Hacker News, but you'd never tell that just by looking at the superficial details…

Echoing this -- find a few landing pages you like, try to copy their design, and then get feedback on them here, from friends, other forums.

Third party cool like what?

Have been on Wordpress for years and just tried Webflow. Really digging it!