How to term those words which are not by default English but are added over time?
Consider for example Google. Is it an English word now? Or is it just a usage?
Use of the word “google” to mean “perform an Internet-based search (for)” is an example of genericization.
Wikipedia has a list of terms that have entered the language this way. Some may surprise you.
Wikipedia also has a helpful page on the topic, which includes commentary on the phenomenon itself.
If you're looking for a more general term that covers words entering the language that are not necessarily trademarks, I recommend neologism.
"How to term those words which are not by default English but added by time?"
That seems to me difficult to answer since all English words were not by default, but added in time. You may consider, depending on context, terms such as neologism, loanword or calque.
As for Google, this is a type of trademark name that became a common word. Other such examples are: cornflake (from Cornflakes@), tabloid, zipper, aspirin, escalator, jacuzzi, ping-pong, and one of the most-known, xerox.
Whilst a lot of the other answers to this question seem to be focusing on whether google has entered the dictionary, to me that sounds like a question for ELU.
My (ELL) take on it is that it is a word - regardless of whether a dictionary believes it to be so - meaning to "search the web (esp. using the Google web-search product)" because native speakers use it for that purpose.
Whether or not it is in a dictionary is somewhat besides the point. It is well understood, routinely used and widely accepted word in everyday ordinary English.
It is certainly very informal, so I'd avoid using it in essays and graded assignments, or in formal letters and emails. And I'd avoid using it in books and marketing material too, since Google is a trademarked term.
But yes, for all intents and purposes, google is a perfectly valid word, used and understood by native English speakers:
Let me just google that for you.
I'm not sure. I'll google it.
Of course I know that zebras aren't just stripy horses! I googled "zebras" earlier, and it said they were entirely different species!
As per the Merriam-Webster Dictionary, google is a transitive verb. It is a word rather than just a usage.
"Verbing" is what happens when a noun or adjective starts to be used as a verb.
"Genericizing" or "genericization" is what happens when a trademark (such as "Google") starts to be used as an ordinary word (such as "googling"). Usually the trademark is capitalized, and usually the genericized word is not capitalized. Also, most trademarks do not take prefixes or suffixes (other than using a plural suffix to talk about more than one instance of the trademarked item), but genericized words do.
"Coinage" or "coining" is what happens when someone makes up a new word (or phrase) and starts using it. The new word or phrase is a "neologism".
"Borrowing" is what happens when a foreign language word becomes an English language word.