What is the time complexity in solving a polynomial equation of degree N in one variable? How easy/difficult(in terms of big-O) is it to solve polynomial equations of high degrees of N?

Do apps like Desmos actually solve equations to get roots or do they approximate roots by iterating through a range of values for x?

What mechanisms are at play in solving a univariate polynomial equation algorithmically?
The time complexity doesn't just depend on N. It's possible that due to the coefficients, you have to evaluate the polynomial with very high precision to determine whether some minimum of the polynomial near a point x has a value >0, =0 or &lt;0, and you need to know this to know the number of roots near to x (0, 1, or 2).