If I am understanding well...file managers mostly title some part of file system as "SD card" despite the fact that it is not SD card, or even sometimes when the phone doesn't have a slot for SD card at all (this happens for example on Nexus 5x). It's not just one or two file manager, but lots, so I think it is more Android like "problem". So I am asking: What is the reason?
This is done so that on a device without a removable SD Card 1 - you'd still be able to copy files to and from the device, and 2 - user's data can be stored separately from system and app data.
Regardless if a specific device has a removable SD Card, Android sets aside part of its internal memory that is reserved for user's media such as photos, music, movies, books etc. That is the area that becomes visible to a computer when you connect the device via USB cable, and is commonly referred to as "internal SD Card" for the sake of simplicity. If a device has a removable SD Card, you will have another area that is visible to a computer, and that is referred to as "external SD Card."
On older versions of Android internal SD Card was set up as a separate partition, and formatted as a variation of Fat/Fat32 type so that it could be accessible as a UMS device. On newer Android versions there's some behind-the-scenes logic happening to expose internal SD Card to computers via MTP protocol instead.
Most Android file managers will show both internal memory, and the external card if one is present.