When the phone specs say "Expansion card slot supports microSD™ memory card for up to 128GB additional storage". Is that 128GB for the RAM (short-term memory) or the ROM (long term memory)
That would be for the built-in non-volatile storage1, not RAM. 128 GB of RAM is not something you will need in a mobile phone for a long time.
1 Strictly speaking, ROM is not the correct acronym because it stands for "read-only memory", which the built-in storage of your phone is obviously not.
A microSD card is by it's very nature flash memory (or, in theory, something that pretends to be--but there are no such alternatives these days)--they didn't specify what type because there was no reason to.
As Indrek said, it's long term memory.