I saw a video where it says immersion is important especially in the beginning(for language learning). So I'd like to know if watching a Japanese streamer(e.g. a Japanese Vtuber) would count as immersion. Thanks in advance!
What Japanese learners on the internet call "immersion", which is just consuming native content, isn't what immersion is to begin with, so don't worry about what "counts" or if you're doing it "right", because the answer is going to be "no" right out of the gate.
Real immersion is prolonged activity in an all-Japanese environment where you have to both understand and respond without using another language, like attending a class where the teacher only uses Japanese, or making yourself understood while in Japan.
Watching videos or reading books can definitely help you learn greatly, but they aren't "immersion", and people on the internet peddling "immersion" as the secret to learning are dishonestly conflating the known benefits of real immersion learning among Japanese speakers with watching anime alone at home. So don't worry what internet people say is the "best" way to learn or what methods you "have to" start as a beginner (personally I would strongly argue AGAINST native content at the beginner level, but everyone learns differently), and just explore different ways of learning and engaging with the language that you enjoy most and work best for you.
My (probably unpopular) definition of immersion is exposure to everything spoken or written in Japanese, and I include anime etc with English subtitles because you are subconsciously picking up the sounds and the rhythm of the language. For the same reason, listening to Jpop, even when you can't fully understand the lyrics, is also immersion in my book and has a benefit. Same with tackling the written instructions on the instant ramen packet. Even if you can pick up just a few words as you consume their media, it's progress and it's all grist to your mill. (Sadly, it won't replace studying though.)
It wouldn't hurt, but it also wouldn't be much of real use at the beginning level. It might help tune your ear to the flow of the natural speech and if it's your source of motivation to study - it's great. Aaand that's about it. Enjoy it in your free time as a way to create an imitation of language environment, but don't count it as actual study time good luck!
Even real immersion is controversial. Many language educators advocate for a bilingual approach, as speaking to students in their native language makes it much easier to explain things in a way that the student can understand and gives the student opportunities to ask questions and clear up misunderstandings.
good luck!