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It is also important to get good habits like keeping the header tag and all of its components under one leading element such as div so that when the header changes size, all of its children elements will too. Bad HTML and CSS structure are usually the cause of these issues. Viewbox too.
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It’s all based on your viewport which is the overall view of the screen you are looking at. It is the same as when you look at a website on a different device(Tablet, phone, etc…). Every website also differs with how the containers on the screen will change depending on the size(RWD, flexbox, grid, media queries). If you increase the screen to 120%, its just like you are viewing it on a larger screen size.
If any of the above terms confuse you, that’s okay, I would recommend looking them up as they will be useful.
Look up viewport CSS and you will get more information on such topics.