Mining centralisation is on the following aspects:

Timechain content responsibility and reward distribution.

The chains in the timechain are created by single entities who determines what it contains. In order to manage income and combine hashing power to collaborate in order to get more (regular) profit, miners started to point their hashpower to centralized entities.

In order to collaborate, all the hashers must join forces on the same thing, so in general miners started to follow instructions of a centralized entity in order to get on one line. This however gave much power to the central entity who can essentially command a hashing network to do what the central entity wants. Including what gets inside the chain.

The second thing is coin distribution and it is heavily custodial for a network that is focused on non-custodial ownership. A miner cannot guarantee his ownership over his income unless he uses a special protocol.

Which brings us to the solutions: there is an unscalable but fully decentralized protocol called p2pool and there is an other protocol called stratum v2 which can distribute the responsibility for managing what is inside the chain. Then there are seperate pools that are as transparant as possible and then there are attempts to make the miner income non-custodial.