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...

You need a transceiver chip which then hooks up to the Ethernet jack.

Usually have to do this for any interface when the signals don't come in right at logic level, like CAN, RS-485...

although it's not always exactly 'just' logic level conversion.

Looks like you need an external PHY. It can talk to the PHY with RGMII which uses 12 pins, which are muxed with GPIO8-19.