I am new to econometrics and tried to find the answer on google, but couldn't find any. I've learned that Fixed-effects models depend on there being variation within each higher-level unit of analysis. For example, if there is no variation within a company's observations, it can't be used in the model.
1. This is probably only for the main predictor variable right? All other control variables wouldn't matter? For example, if company size is the main predictor and I use size fixed effects, if there is no variation in size then STATA will automatically drop those variables?
2. Is this also relevant for continuous variables?
3. In a fixed-effects logistic regression model, you cannot use observations that have no variation on y. Why is it the y variable here not x?

These might be very simple questions for most of you.. but would appreciate if anyone can help!