Sort of. And this "viral" nature applies to all copylefted licences (GPL1, GPL2, GPL3, AGPL, CC-SA).

You can take GPL3 code, use it, and then close off your own source, as long as it's for private purposes. What you can't do is then distribute it under different terms. The GPL says that if you take GPL code, modify it (make a "derivative work"), and then redistribute it, you must redistribute it under the same terms that you got the original code from. So your users must have the same rights that you had when you got the original code.

this is the "virus" thing explained (by chatgpt, heh, but this is exactly what I heard) : Inheritance and Derivative Works: In object-oriented programming, inheritance allows you to create new classes based on existing classes, inheriting their attributes and behaviors. When using GPL-licensed code, whether GPLv3 or LGPLv3, any derived classes or subclasses created within your project will be considered derivative works. As a result, if you choose to distribute or publicly release these derivative works, they must comply with the licensing terms of the original GPL-licensed code.