Openings have evolved for centuries and matured into what they are today. There are many ideas behind each line. Every move is played for a reason.
There is a reason why it is called theory. It is not set in stone but there is a consensus until someone tries to challenge it with a new idea.
Also there is the situation, which you will encounter, where you play other beginners who have been studying a different variation or system than you and will play different moves than those you know. So in a matter of a few moves you will both be “out of the book” and on your own.
Your time is much better spent training tactics and practical endgames and going over the games you play to see what went wrong and right. There is a special training that is very helpful not just to beginners.
Playing lines over in your head as far as it goes, without moving the pieces on the board. In books there are often variations to the moves of the main game/combination. Try to “see” them. In the beginning it will be extremely difficult. Go back and try again if you lose your train of thought. After this special training it is often nice to play out the moves on a real chess board.
Start with 5-15 minutes a day. It is better to train a little daily than once a week for two hours. Already after 1-2 months of this daily training you will notice huge improvement in what you see over the board. Well, I did and I’m “just” a hobby player.
For the beginning chess player, there’s a lot of emphasis mentally on starting out in the game correctly. The problem is that it’s hard to find any advice on the opening besides very basic tips.