How can anyone possibly predict how the price of anything (stocks, Bitcoin, fiat, etc.) will change without "insider knowledge"?
How can anyone possibly predict how the price of anything (stocks, Bitcoin, fiat, etc.) will change without “insider knowledge”?

You can do that by forecasting based on data.  Exact forecasting model will depend on exactly what you want to forecast but most economic variables can be forecasted with a degree of accuracy that is generally higher than just 50/50 random chance (see Elliott &amp; Timmermann 2013 Handbook of Economic Forecasting).
Some series are more difficult to forecast,  for example it is very difficult to beat random walk when forecasting forex exchange rates (Kilian &amp; Taylor 2003), but even in those cases it is not considered impossible only exceedingly difficult.
When it comes to bitcoin forecasting specifically you might want to inquire more about exact models used there at either Quantitative Finance or Bitcoin stack exchange, but just casually search of google scholar reveals plethora of models for forecasting bitcoin returns or trends in Bitcoin prices. For example, Xiaolei et al (2020) report relatively accurate forecasting of Bitcoin value trends with Light Gradient Boosting Machine.
In addition, you would likely want to have not just once forecasting model but multiple ones and analyze their forecasts together.