In the US, at least, usually if you’re asked about your ethnicity (say when you’re doing the common app for colleges) it means that you are at least a quarter (I think, maybe it’s an eighth) of whatever the ethnicity is. So for example, my dad was a Hispanic immigrant, which makes me 50% and therefore ethnically Hispanic despite living in the US my whole life. Honestly, I don’t have much in common culturally with Hispanic people who grew up in a Hispanic country, however I have a fair bit in common with other Hispanic kids who grew up in the US, distanced from our parent’s culture. Sure the Wikipedia definition may say that that’s just one way the word ethnicity is used, but if you just go ask people “hey what’s your ethnicity” aside from very likely being confused why you’re asking, they’ll likely answer something like hispanic or Asian or whatever

I think culture is the better word to use instead of ethnicity in this case. Like others have said though, the internet is such a big place, that it’s not very descriptive to say your culture is internet culture. Even in your original comment, you had to clarify that you’re referring to the western/english side of the internet, and even that can be broken down further. Do you primarily use TikTok or Reddit? Instagram or Facebook? Do you collect memes or read fanfics? Do you play video games or watch craft how-to videos? The internet is vast and obviously you have more in common with the people and content you find on the internet because the whole point of the internet is to be able to find people like you. But there’s a whole bunch of other people out there using the internet that have nothing in common with you, like for example, your neighbors. It’s not wrong for you to say it, it’s just not particularly descriptive without a bunch of extra info