Sd card is an external memory card. There is also a big TF card, which is bigger than a small sd card and has the same function as a small sd card. Their external interfaces are different. You can insert a small sd card into an empty TF card holder, convert the small sd card into a TF card interface, and connect it with a digital device with TF interface. Small sd card is suitable for the socket of mobile phone or digital player with small sd card bracket. Through this empty TF card holder, a small sd memory card can be easily converted into a large TF card for use. The two cards are basically the same, but the interfaces are different and the sizes are different. Some TF cards are equipped with write protection switches, which is more convenient to use (see attached figure).
There is an sd card rack in the mobile phone, so you can buy a small sd card to expand the storage space. With sd card, you can store videos, photos and other files.
Some card holders need to open the back cover to install the sd card, and some card holders have sockets on the side of the mobile phone, which can be directly inserted into the TF card.
Android phones have this advantage and are easy to use.
Some Apple phones and iPad don't have sockets, so they can't insert SD cards, which is inconvenient to use. There is a special lightning storage through USB port, which can be inserted into USB port to expand storage space. Not very convenient.