> I don't know exactly why Microsoft chose to combine a browser and the file explorer.
I can tell you. It was clear at the time but that time was 27-28 years back.
Microsoft didn't like it when anyone else made big money off the PC platform, and it got jealous when anyone started "making bank" from tools that MS didn't offer.
In the early to mid-1990s Netscape made hundreds of millions from its eponymous, industry-leading, rich-media-capable web browser (codenamed "Mozilla".)
Every PC and Mac had Netscape on it. It was proprietary, free for personal and non-commercial use -- but corporates had to pay to license it. And they did. In the early days of the WWW, Netscape was the browser.
Whereas when Windows NT (1993) and Windows 95 (1995) launched, they did not include a web browser at all. Bill Gates' circa 1995 book The Road Ahead barely even mentions the Internet at all.
Instead Win95 came with a client for the proprietary Microsoft Network, which extended the Win95 desktop, called "Explorer", and it also included clients for Microsoft Mail -- right on the desktop -- and Microsoft's proprietary chat protocol.
An optional extra, "Microsoft Plus!", included a fairly poor web browser, bought in from Spyglass and rebadged Internet Explorer.
This was a £40 add on to Win95.
Netscape made a killing. Microsoft got angry and wanted revenge. In one leaked quote it wanted "to knife Netscape in the back."
So it started offering IE as a free download for Windows 3.1, Windows NT, and Windows 95. It started a relatively rapid development programme to improve it. IE 1 was very poor, and IE 2 wasn't much better, but IE 3 was all right.
This angered a lot of people.
1. Competitor launches product
2. Product gets successful, makes lots of money.
3. Microsoft pours money and effort into making a rival product -- OK, fine.
4. MS makes it free, and offers it as a free upgrade for existing users. Not fine.
Then came IE 4. This was launched with a big splash. It was built into the shell.
Instead of simply showing folders, the Explorer now rendered their contents as HTML and used the IE engine to show them. The desktop backdrop was Web content and could change through the day. You could pin a Web bookmarks bar to the Taskbar, or have it floating. There was also a floating list of web thumbnails and shortcuts. The .HLP help file format was replaced with new HTML help. Optionally, icons became web links: name underlined, and opened with a single click.
This new "improved" desktop was offered as a free upgrade for Win95 and NT 4.
And a year or so later the new "Active Desktop" was bundled as part of the new Windows 98.
There is a single functional improvement in AD that was not connected with shoving everything possible through the browser engine: it was multithreaded.
In Win95 and NT4, if you start a file copy or move, the shell locks up until it's finished. You can't use other windows or start programs. In Active Desktop, you can: the file operation trundles along and you can keep working. That is the sole non-Web-related difference.
This kind of behaviour is illegal: it's called "restraint of trade". You can't just make a free rival to a competitor's paid one, bundle your rival app so everyone gets it like it or not, and simply get away with it. That's anti-competitive behaviour. So MS made its developers go out of the way to find ways to integrate IE4 deeply into the Win98 desktop so it could claim it was an OS component and not anti-competitive bundling.
Netscape complained to the US government. The US government acted.
But after years of fighting, MS got off Scot-free. Netscape collapsed and was split up and sold off. AOL got the browser (but used IE as it had a secret back-room deal with MS), Sun got the web server, and the unfinished unreleased next version of the browser was made FOSS and a new non-profit foundation set up and named after the browser's internal codename.
https://www.mozilla.org/
All those bolted-on shell extras to justify the presence of a web browser? That is the design KDE copied, not the original, much smaller and faster Explorer desktop.