Greeting Nikola, In my opinion, these are things that you should take into consideration:
Instead of using p1,p2,p3...,you could use Descendant Selector for example: div p { } to style all the p tag inside div or .class p {} to style p inside that class. You can read more here : w3schools.com/css/css_combinators.asp
I'm not quite understand what purpose of this line div style="display: flex;", maybe you want it to display inline, you could set a class name for that div and then use Descendant Selector, example: .class p { display : inline } instead
I see that there's many p tags have same attributes, instead of writing for each tag, just write for one p tag, and it will apply all the p tag in the page, this basic technique can use for all tag in html file, you can read more about it here: https://www.w3schools.com/css/css_selectors.asp
Instead of ".button" class, button tag might be a better option you can check out more tags here : https://www.w3schools.com/tags/default.asp, similarly .header -> header tag
You should be aware when using inline styling because according to Cascading Order, the inline style takes the highest priority, which mean it's useless if you want to change some attributes in external stylesheet for example in your code :div class="whyUs" style="margin-bottom: 30px;> if you want to change margin-bottom : 100px in .css file, it will not be applied you can read more about Casading Order here : https://developer.mozilla.org/en-US/docs/Learn/CSS/Building_blocks/Cascade_and_inheritance or example here : https://www.w3schools.com/css/tryit.asp?filename=trycss_howto_cascade
Font-size and font-family should in body tag, if there's just 1 font so you don't need to rewrite multiple times
That's all, these are just my recommendation to optimize some part of your code, you didn't do anything wrong, these advice are based on my experience, i'm not judging anything,okay :))
In general, if this is your very first project, you are doing extremely great, you even know responsive design, better than me back then :)), good job
Hope this help, Happy coding !
