According to the paper, in a modular stack, the sequencers act as both builders and proposers of blocks to the underlying layers they are connected to. The validators on the data availability layer are purely proposers and are unable to reorder any transactions.
EigenPhi has captured Block Builders' address, and discovered the top 2 capturing over half of MEV after the Merge. It's easy for builders to manipulate in a monolithic blockchains rather than modular ones.
The total MEV of these shared layers are in general (beyond fees) the total amount of inefficiencies in the system caused by non-coordination between actors in the supply chain (liquidations, arbitrage, front-running, sandwiching)
With a much less congested network, arbitrage opportunities on BSC were much more cost-effective than on ETH. As blockchains grow more efficient like the paper suggests, the difference will eventually converge.
On thing to be noticed, however, is that, despite the fact that MEV can happen on different chains, MEV searchers and bots are the most active during crisis. Yes, crises breed MEV for its drastic price volatility