Fidelity does this because they invested in Twitter alongside Musk, and as a bank they are required by law to disclose the current value of their assets. They come up with a value of their stake, then the rest of us divide that by the percentage Fidelity owns to get the value of the company.
Companies such as Fidelity give an estimate on how much they are value after analyzing the finances of the company I think.
Fidelity does this because they invested in Twitter alongside Musk, and as a bank they are required by law to disclose the current value of their assets. They come up with a value of their stake, then the rest of us divide that by the percentage Fidelity owns to get the value of the company.