Sharding of Ethereum POS is a good example of modular Blockchain
Modular Blockchain has been both scalable and decentralized, this was only possible because they shared the task in to three sections or layers which is Execution, consensus, and data availability.
Moreover this system is organized in a way that makes the blockspace bigger and setting more focus on shards, this makes modular Blockchain unique that monolithic Blockchain.
Let's discuss two of this layers.
Execution layer
Execution helps modular Blockchain to increase throughput, they do not just place L1(layer-1) where the transaction are executed instead it is been shared between L1 and the rollups, so rollups now complement the execution for L1.
Data-Availabilty layer
Sharding helps to create an accurate data in time for example once Ethereum data layer shards the validator in the network will escalate to smaller network where transactions on the Blockchain will be verified thereby creating more blockspace in the chain.
This process ensures that transaction history behind each block header is made available and accessable to anyone who wants to check the transition details.
why do we need modular Blockchain
Most importantly modular Blockchain helps to solve major challenges that monolithic Blockchain can't handle like checking consensus and validation of transactions in the Blockchain.
Modular Blockchain do not deprive other technology in the Blockchain to perform the duties they are situated for.
The separation of resources in modular Blockchain helps increase scalability in the Blockchain because each layer work effectively on its specialization, this is very important because it increases throughput thereby giving users more assurance especially first class users of the network.
Modular Blockchain is flexible, users can do thier module upgrade anytime depending on the advancement of the technology and thier personal needs.
In summary modular Blockchain through its distribution of various task helps the Blockchain security and also increases the Blockchain by increasing the blockspace in effect trying it's possible best in solving the Blockchain trilemma.