Ethernet PHY clock and sync
Its clear that for any serious communication, synchronization is needed. But in which way is the PHYs external reference involved, since free running clocks also work and therefore why do some applications need a synchronous clock between multiple PHYs or the MCU (the MII already has its own clk)?

PHY's (Physical Interface IC's) and clocks can be confusing, there can be several clocks associated with one PHY.

The PHY has an internal clock generated from it's oscillator (or external source with some PHY's). Some PHY's also provide an option to pipe out their clock, but are not essential to the MII interface.

The MII has it's own data clock or clocks. It can have one for TX data clocking and one for RX data clocking, this is only for data. The MII clocks run at 25 or 50MHz. RMII uses only one clock for both directions.

The MDC is a clock for the MDIO interface which is a control interface that sends commands between processor and PHY.

Ethernet itself does not need a clock for synchronization (but does have a protocol for clock synchronization)