The Bitcoin mining network consists of many mining companies that combine their hashrate in various mining pools. To estimate how decentralized the Bitcoin network is, we can look at Bitcoin's hashrate dominance by mining company or mining pool.
We start with the mining companies. Several publicly traded mining companies have grown to control a significant share of Bitcoin's hashrate. The largest publicly traded mining company, Marathon, generates almost 2% of Bitcoin's hashrate. In total, the five biggest publicly traded miners control around 7% of Bitcoin's hashrate - a negligible share.