When you manually install WordPress, somewhere during the process it asks you to select weather your site uses http or https. Why is this? What does it matter? How will your choice translate to changes/a different use of eg your database? Bc when I installed WP I chose http bc at that time I didn't had an SSL yet. Then I created a few pages, posts and categories. The URLs of these started w/ http. Then a while later I installed SSL certificate on that domain name and since then pages, posts and categories I created, their URLs, started w/ https. Can't remember if I also had changed http to https in WP Dashboard's settings then. Is all that choice for http or https (during installation of WP) does, is determinning weather or not to place http or https infront of the URLs of pages, posts and categories you will since then create? Then where does WP store these URLs (are they called 'links' BTW)? In the database? In which table(s)? How does this work?
An understanding of how WP stores content is important. Pages/posts' content is stored in the database. Media files are stored in the uploads folder, and the pointers to the media (their URLs) are stored in the database. (Media files are just another type of post/page.)
The base URL for your site is stored in the wp-options table, in two places. That is what is set up during the install. And those two entries are the most important places for your URL.
But if you already have existing content, and you add HTTPS, then you need to change all of the places in the databases where your URL is stored. If you don't, then the visitor's browser will show 'mixed content' warnings, a message that some of the content on your page is HTTP and some is HTTPS. That can affect visitor's perception of your site.
So, if you update to HTTPS, then you need to adjust the URLs in your content. Although you can do this manually (via myPHPAdmin or similar), I have found it easiest to use a plugin like "Better Search and Replace" which is quite effective and fast in adjusting URLs to HTTPS. (It is also good if you move your site to a new domain.)
As for why HTTPS, as mentioned, modern browsers will flag your site as 'unsafe' if you do not have HTTPS. With the availability of free SSL certificates (and many hosting places that provide SSL certificates for free), there is no compelling reason NOT to change your site to SSL. This can be done during the initial install of WP, or later. If later, then changes to URLs in the database are important, as mentioned above.