Screenshots and software data downloaded from Nintendo eShop can both be saved on a microSD card inserted into the console. However, downloaded software data can't be transferred to another microSD card using a computer. In order to save software data to a different microSD card, you'll have to insert that microSD card into the console and then redownload the data from Nintendo eShop.
Screenshots will be saved to the "NintendoAlbum" folder on the microSD card. You can transfer screenshots from the microSD card to a computer. Please be aware that the Nintendo Switch console won't be able to recognise the screenshots if you change the file or folder names or the folder location on the microSD card.