In the early 90s all the "cool kids" (for a techie definition of "cool", i.e. hackers) at my University (a Technical one in Portugal with all the best STEM degrees in the country) used Linux - it was actually a common thing for people to install it in the PCs of our shared computer room.
Later in that decade it was already normal for it to be used in professional environments for anything serving web pages (static or dynamic) along with Apache: Windows + IIS already had a lower fraction of that Market than Linux + Apache.
If I remember it correctly in the late 90s RedHat started providing their Enterprise Version with things like Support Contracts - so beloved by the Corporates who wanted guarantees that if their systems broke the supplier would fix them - which did a lot to boost Linux use on the backend for non-Tech but IT heavy industries.
I would say this was the start of the trend that would ultimately result in Linux dominating on the server-side.