In the normal decimal number world, numbers are numbers and letters are letters except in places where we may call letters numbers, like in a car tag number.
In the computer world (and in mathematics but that is not my direction here) several number systems are commonly used. When things are generally presented to humans usually it is in common letters and numbers.
In a few situations you may as a general user encounter letters that are numbers. Some things are shown to humans using the hexadecimal number system in addition to the normal usage of decimal. In addition, the computer works internally using binary, but normal users seldom see things in binary.
MAC addresses on network devices are expressed in hexadecimal. IPv6 (the protocol used for addressing and routing on the Internet uses hexadecimal0. IPv4, which is still used, expresses addresses in decimal.
MAC addresses are network addresses that are burned in the device at time of manufacture. It is a 12 hexadecimal digit number with the first half identifying the manufacturer. Every address is unique. If you look on any device that connects to a network, you will see a MAC address. These may look like 00000C-12D7F2 or 00-00-0C-12-D7-F2. The address is in binary in the machine but displayed in hexadecimal to make shorter and easier to read.
An IPv6 address could look like 7600:10b5:b062:61e4:74d7:f292:802c:fbfd. You will see some of these sometimes shortened where they eliminate repetitive zeros. That address is a number, it is a hexadecimal number. Hexadecimal has 16-single digit numbers 0, 1, 2, 3, 4. 5. 6. 7. 8. 9. 0. A, b, c, d, e, f. The letters are expressed as upper or lower case.