Bitcoin is often referred as "digital gold" due to it scarcity and decentralized nature.because their will only be 21 million Bitcoin in existence which gives it scarcity and makes it resistant to inflation.this limited supply to achieve through a process called mining,where new Bitcoin are created at a decreasing rate over time.bitcoin is decentralized because it operate on a network called Blockchain,which means it's not controlled by any central or government.similary,gold is not control by any entity.
Bitcoin is not digital gold, It's just an expression because gold is considered to have a high value, and bitcoin also has a high value. The initial vision of bitcoin was digital currency, but today bitcoin is also used as an asset. Maybe on this side there are also similarities between bitcoin and gold, gold used to be a currency before being replaced by a fiat money system, now gold is no longer a currency, just an asset. While bitcoin still has both, it can be used as a currency as well as an asset
One of the differences between gold and bitcoin is their supply. Gold has a (almost) unlimited supply because gold will always be mined every day, whereas bitcoin has a limit of 21 million. After 21 million bitcoins have been mined, there will be no more bitcoin mining. So maybe bitcoin will be more than gold