If you want to invest in a long term profitable asset and you don't have a discretionary fund you can do the following. Cutdown on your basic expenses if you can, get a better paying job, get a side hustle, learn profitable skills, do what you can to earn more than your basic expenses.
It may not be reasonable for a person to reduce fundamental expenditure, especially for investment because basic expenses refer to the minimum expenses that are essential for everyone to live. Since we need discretionary income to invest in Bitcoin, stocks or real estate and gold, it would be reasonable to exclude unnecessary expenses from our monthly income. You are right that if someone's total monthly income is equal to their basic or necessary expenses, then it will not be easy for them to engage in investments, because then there will be no discretionary income, which will result in them not being able to save money. Therefore before making any investment, one must first be vigilant about increasing the source of money and must raise capital, and it will be possible to increase it easily by doing side jobs and learning profitable skills. Even then it may be somewhat difficult to reduce spending because inflation can create pressure and everything has become more expensive.