Internal storage, a.k.a. adoptable, essentially forces you to always leave the microSD card in your phone. It becomes integrated into your phone's internal storage memory, that's why it will be reformatted to the proper file format and because it's also encrypted with the encryption key tied solely to your phone the card is unreadable if you should remove it. (Note you can always re-use the card by just re-formatting it, returning it back to its original state. But you do need to go through the proper procedure to change an adoptable card back to a removable one as your phone and the operating system were working with what was once that additional memory storage space.)
As for 'portable', by default a microSD card will be formatted using a variant of the FAT file system (Microsoft's File Allocation Table), maybe FAT32 probably exFAT. Note that FAT is dated, no longer actively supported, and proprietary, with history of being unstable to begin with, so cards are often easily corruptible. The plus being the FAT file systems are supported by Linux, Macs, and of course Windows so portable cards can be used to physically transfer media between phones and computers, with the caveat that FAT isn't capable of handling the same file and folder file permissions or file metadata that Android relies upon very well so while files themselves are easily transferable, it's not always seamless.
basically insert and leave it.
