Not "just", it's (presumably) 8 dedicated pins that form an RMII interface. This is not the same 8 pins as you'll find in your 4-pair Ethernet cable, it's a separate protocol which can be connected to an Ethernet PHY transciever like a TI DP83867E [1], which is further connected to "magnetics" [2], a convenient package of 8 integrated transformers and chokes that provide the galvanic isolation feature of an Ethernet connection.

A few SoCs provide integrated PHY transceivers, but usually it's an external chip.

[1]: https://www.ti.com/lit/ds/symlink/dp83867e.pdf

[2]: https://yageogroup.com/content/datasheet/asset/file/DATASHEE...