There's no simple answer.
How data is distributed across multiple lanes depends on the exact physical layer variant. With 1000BASE-T, possibly the most popular multi-lane variant, the four five-level symbols transmitted simultaneously form a four-dimensional tuple (4D-PAM5). Each symbol at 125 MBaud effectively transports two MAC bits, resulting in an ...
The IEEE standardized Provider-Backbone Bridging (PBB), also known as MAC-in-MAC encapsulation, as 802.1ah, which has been integrated into the 802.1Q standard.
PBB is, aside from 32 bits service encapsulation, basically plain Ethernet frames as a payload of other Ethernet frames.
A typical PBB frame looks like this:
0 1 2 3 4 ...