Note that Samsung has disabled adoptable storage which was introduced in Android 6. Which means the micro SD card won't be able to extend the internal memory to store app data etc, only can be used to store documents, photos, videos.
Also, for the galaxy S10 series when you record 4K 60 FPS video the phone will auto store into internal memory even if you have selected the storage path to be micro SD card. So have to manually transfer over every time. Needless to say 8K video recording on all the S20 series will likely be written to internal memory as well. For photos no issue. But for the 108 mega pixel photos on the S20 ultra if the file size is really huge maybe Samsung may also default to store it into internal memory.
That's why internal memory storage is still always king. Works 100% of the time and more reliable for frequent read and write. Micro SD cards are known to be more prone to memory corruption. Really dunno why this time Samsung bring in 128 GB S20 ultra totally makes no sense. Those who can afford to buy this model will be able to pay for higher storage.