I am going in the gym for some time and I notice that while I improved my strenght and resistance a lot (I can lift heavier weights and for more repetitions) but I don't notice them growing. What am I doing wrong?
Strength is faster and easier to measure than muscle growth. Especially starting out, you can notice changes day-to-day or week-to-week. To measure muscle composition, you need to take a measuring tape and measure different points of your body and record over time. This can be tedious and most people don't do it. You can also take progress photos which is less tedious, but gives you less data.
Muscle growth is done slowly over time so you get used to your look. Similar to how you don't notice a child growing taller if you see them every day. You see yourself every day when you look in the mirror or look down, so you don't notice changes. If you took pictures of yourself six months ago and compared to now, you'd probably notice some changes.
Muscle growth is obscured by body fat. Lean people can typically see changes in muscle composition. People with moderate body fat levels (greater than 13% in men) will need to lose the body fat to see the muscle definition.
Adding on to what DeeV said in the prior answer, remember to focus on the eccentric phase of the lift if you want to maximize size growth.
For example, doing a deadlift and dropping the bar from a locked out position is the wrong way to go about getting growth gains. You lose more than half of the potential growth by doing this.