Each new player has level 1 on each sphere. Level number - quantity of tools of the same sphere that can mine resources at the same time.
For example, if you upgrade the OIL sphere to level 2, it means that 2 oil tools can work in parallel.
Prices for upgrading are the same for each sphere. OIL sphere upgrade price means the number of oil tokens etc.
An upgraded sphere also increases the player's profit from mining. Sphere multiplicator just like season index impacts final profit by multiplying season-based reward.
Premium/Silver pass also multiplies it by 1.5/2.
For example, if you have a 10 sphere level and staked premium pass, the total sphere multiplier will be equal to 1.36, since 1 + 0.18 * 2 = 1.36, with a silver pass it would be equal to 1.27