Think of it like cars. They all drive you down the road, but each car is a little bit different.
Some are 4WD and will perform better on hills/etc. Some may have special grip tires. Some run on electricity. Some just look fancier than others or have bells and whistles not available in others.
Browsers are the same way. All browsers at their core are created with the idea of showing web pages in all their glory, but they are all just a bit different.
Some have different “engines”. (Literally called a browser engine, responsible for much of the differences you describe), others will include extra features not necessarily supported by others (add-ons, extensions, etc.)
Internet Explorer was one of the first browsers and had several specific quirks and features that became relied upon over its long history. Some of those “features” introduced security issues that were not picked up by other browsers.
(There’s a lot more to internet explorer and its sordid history in the browser wars, but this should work for an overview-type answer.)
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