Yes, as a general rule they gain nothing from screen time at this age. It represents only 'opportunity cost'. That is, they lose out from the things they could have been doing instead. They can't understand what they're seeing and hearing, so they have no way to contextualise it and save it into their brains as useful knowledge.
Some people are arguing that IQ is inherited and can't be changed, but that's a little inaccurate. Your genes give you source material but nurture is what 'makes the most of it' and allows it to develop. A tomato seed can only ever grow into a tomato plant. But how big and fruitful it is, or if it ever grows at all, is heavily dictated by its environment. The genes may limit or increase the potential for fruit and size etc but a poor and unhealthy environment will do far more damage. So that's kind of how it is with humans or other animals. You can't make them 'more' clever, you can optimise what they already have. You're probably aware that at birth babies still have a huge amount of brain growth left to do.
Safety, love, play (chat), good food and exercise.
Stress (feeling unsafe) and neglect will limit brain development and contribute to developmental delays, mental health problems etc. Google the Romanian orphans to find out more.
Make sure your baby always feels loved and safe. That is the most important thing. Frightened animals (and people) cannot learn as well because the brain will ALWAYS prioritise safety first. This includes practicing a level of radical empathy that is lacking in many parents. (not spoiling the kid, but understanding them and explaining things).
Talk to him all the time - try to take him everywhere that is safe and appropriate, and chat about the things you see. They understand a lot long before they can speak and it wires their brain to your language and feeling loved. It develops an active, thinking, mind.
As he gets older try to explore lots of different things. Don't expect your kid to have the same talents and interests as you. They might, they might not. You need to figure out what they were born to love and are naturally good at (or at least happy to spend lots of time doing) and make opportunities and give gentle encouragement (not pressure). Back to the tomato plant - we give it fresh air, water and sunshine, and let the plant do the rest. Oh well then we add some support poles when needed.
If you want a child that grows up into someone who can think things through:
Read up about the Socratic method and try that. It's basically asking questions and getting the child to give their ideas.
Never ever ever make fun of him for his questions or theories.
Let him make mistakes. As long as it isn't life threatening/seriously harmful, mistakes (in a safe and non judgemental environment) are an incredibly valuable learning tool. Yes, even at six months as they start to explore their environment you can let them try things you know won't work.
In a similar vein, responsibility. Let him have responsibility for anything he can handle. Assume he can handle it (age appropriate of course) before taking over and diving in.
Critical thinking is a good intellectual skill. You can help develop this by modelling it and talking to the child, asking questions etc. You will probably want to use google for better info.