Now, why it's like that ... we have to go back to the early days of android when phones had very little memory and no storage. But, they all had an SD card slot, so if you wanted to expand it's capabilities you just put in an SD card. You must understand that Android is based on Linux and because of that, certain file system conventions are similar. In this case we are talking about mount points. That's where the OS knows where files are located. We start with root which is represented as "/". from there you begin your folder/directory structure. For app developers to take advantage of sd card storage there had to be a common standardized mount point, otherwise a developer would either have to account for all phones, models and configurations (which probably wouldn't be possible) or ask the user to set the location at installation which defeats the purpose of easy interchangeable apps and services. So it was determined that the SD card would mount at "/sdcard".
This is where ALL apps look for storage space. Fast forward a year or two and phones with 4GB, 8GB and even 16GB of internal memory started showing up in the stores. Android needed space for the system and for apps, so the rest was used for storage, which Android expected to be an SD card. Manufacturers mounted the internal storage at "/sdcard" which worked just fine. If those phones also had a slot for an external SD card, then it got mounted someplace else. And, because not all phones have an SD card slot, there was no standard set, so it could be anything. It's usually something like "/sdcard/external" but you'll see enough variations so that all it can be used for is file/media storage. Not an elegant solution, but it worked for most people.
Still, it caused problems as apps and games required more and more. Today you'll see phones with 32GB and 64GB of internal memory, and those that have SD card slots can support perhaps an additional 256BG of storage. With all this memory there had to be some advancement in how it's used. Newer versions of Android (6.x and later) off you a choice of how you want to use it when you insert an SD card. You can select /file/media storage only, which means pretty much that. If you have a huge music library, download videos and take tons of pictures, this is ideal. The second option is adoptable storage. What this does is use your SD card as an extension of the internal memory. The card is encrypted and tied to the phone so that you can't remove it without reformatting it and losing everything stored on it. With file storage only, you can pop the card in and out without consequence.