We use “on a website” because we treat all technology with screens as surfaces. We use the preposition of place “on” with surfaces in English. Everything on the internet like websites, apps, and platforms will use the preposition “on”.
I saw that news story on a website yesterday.
Where is the online class? It is on his website.
On a Website
We use the preposition “on” with all technology that functions with screens. This is because the images on these screens appear “on a surface” and therefore always use the preposition” on”.
We use “on” with surfaces as it shows the relative position of something. Think about an easy example: “the book is on the table”. The position of the cup is supported by the table. In the case of the internet(and everything that is part of the internet), the screen supports the technology.
For the reason explained above, we use “on a website” in most day-to-day uses on the internet.
In a Website
“In a website” is not a common expression that people use when they are talking about using a website as a user.
There could be some instances where “in the website” maybe be possible especially when we add extra words like “code” or “folder”.
There is an error in the website’s code.
The file must be in the website’s program files.
Other uses of “In” Online
Although “on” is the predominant preposition used with words connect to the internet, you can use “in” in the following situations:
In the online store
In an online course
In an online class.
We use the preposition ”in” with volumes so think of the “online store” as meaning within the boundaries of the store.