Mining was once possible on an ordinary computer. For the largest networks that has not been true for many years, and understanding why explains what remains possible.
Why bitcoin mining is closed to individuals
The network adjusts difficulty so blocks arrive at a fixed rate regardless of total computing power. As industrial operations joined, difficulty rose to match them.
Competing requires purpose-built hardware, electricity at industrial rates, and cooling. An individual paying residential electricity prices cannot produce more value than the power costs.
The exception is someone with genuinely free or near-free electricity, and that is a narrow case.
What the calculation looks like
Hardware cost, divided over its useful life before newer hardware makes it uncompetitive. Electricity at your actual rate, which is the dominant ongoing cost. Cooling, noise, and the space it occupies.
Against the expected earnings, which depend on difficulty and price, both of which change unpredictably.
Most published profitability calculators omit hardware depreciation and difficulty increases, which are the two factors that determine the outcome.
Networks where it differs
Some smaller networks are designed to resist specialised hardware, so ordinary graphics cards remain competitive. Rewards are correspondingly smaller and the value of what you mine is more speculative.
The staking alternative
On proof-of-stake networks there is no mining. Participation means locking up currency, which requires holding it rather than buying hardware. Different risks entirely.
The honest summary
Not viable at residential electricity prices for the major networks. Anyone selling equipment on the basis that it is should be treated with suspicion.