Coronavirus typically has a spike that starts in December and lasts until February. There are many theories about why this happens, but reduced Vitamin D levels has substantial proof behind it.
But yes, there was a spike in December 2019, and another spike in December 2020. So we all are expecting a spike in December 2021.
No, just a significant minority:
https://www.webmd.com/cold-and-flu/cold-guide/common_cold_ca...
Corona viruses are a family of viruses that extends far beyond the particular virus responsible for the current pandemic/epidemic.
Corona viruses are the #2 family responsible for the common (viral) cold, behind rhinoviruses (In the ballpark of 10-20% and 50-60% respectively, iirc, but definitely rank #2 & #1).
So there is a great deal of historical knowledge about the corona family of viruses, just not necessarily specific to this corona virus and it's variants.
In the context of looking for seasonal patterns in respiratory viruses, it's the first place you'd look in trying to find common traits.
ETA: WebMD says 10-40% Rhino, 20% Corona, 20% RSV/parainfluenza, and 20-30% "other".
See for example:
"Epidemiology and Clinical Presentations of Human Coronavirus NL63 Infections in Hong Kong Children"
Authors: Ting Fan Leung, Chung Yi Li, Wai Yip Lam, Gary W. K. Wong, Edmund Cheuk, Margaret Ip, Pak Cheung Ng, and Paul K. S. Chan
ASM Journals, Journal of Clinical Microbiology, Vol. 47, No. 11
https://doi.org/10.1128/JCM.00832-09
https://journals.asm.org/doi/10.1128/jcm.00832-09?permanentl...
In my country - where for the last month we've basically had no sun, just clouds - there is a distinct spike around winter, which would follow nicely with the vitamin D theory. However in places like Southern Europe, which still has a good amount of sun during winter, there are similar spikes.