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Today, I researched how people can vote 21 BPs in the EOS system, so I am wondering, is EOS fully decentralized or does it still use a server center?

EOS does not have a central server or a single point of failure.

All 21 block producers can be replaced and there is a reserve pool 100 block producers waiting at any moment.

Here you get a geo map of active producers https://www.eos-radar.com/

DPoS (the consensus Algorithm of EOS.IO) is a trade-off between full decentralization and speed.
While PoW algorithms allow almost all nodes on the network to create blocks, DPoS gives this privilege to selected Block-Producers (BPs). These are elected by the community. 21 in EOS that are active and 21 in stand by if some BP fails.

In theory BPs should be heterogeneous and geographically distributed. There are several guidelines how to select "good" BPs. Also the community profits from BPs that have their own servers. If all the Nodes are setup on the same cloud e.g. AWS there would be less decentralization.

The actual state of decentralization depends on the active BPs. You can check them e.g. here. Almost every BP has a website or information why you should vote for them. Also reddit has good information about trustful BPs and the latest activities.

However DPoS is often criticized for the whales (people/institutions that hold many tokens - voting power). They may vote multiple BPs they like or they participate with into the pool. Nevertheless the technology is able to provide a good grade of decentralization.

EOS and other sister chains such as TLOS, which launched this week, are DPoS based blockchains. Delegated Proof of Stake (DPoS) can be easily confused as Decentralied Proof of Stake as the acronyms are same. A DPoS consensus-based blockchain puts a lot of "trust" in the operators -- aka Blockchain Producers (BP) -- that run the actual computer server network consisting of block production nodes among other things. These BPs are voted by token holders.

In an ideal world, there won't be big token holders -- also called whales -- who can weigh heavily in the voting process. This turned out to be not the case of EOS. EOS suffers from huge whale problems, and also the voter participation is not ideal. But that has nothing to do with the EOSIO technology that it is built on. Time will tell how the voting and governance plays out on EOS. TLOS, a new chain based on EOSIO software is the "refined" version of EOS by many counts. For example, they have curved the whale problem at the genesis level by capping the max token ownership to a small number of tokens. Again, only time will tell how governance component of these chains works out!