Yesterday, I downloaded a lot of movies and yYoutTube videos because when I return to Japan, I will fly for about 16 hours in total, so and I don’t want to be bored ion athe plane. Yesterday, I downloaded a lot of movies and YouTube videos because when I return to Japan, I will fly for about 16 hours in total and I don’t want to be bored on the plane.

I tried to word this in a way that required fewer commas

Yesterday, I downloaded a lot of movies and youtube videos because when I return to Japan, I will fly for about 16 hours in total, so I don’t want to be bored ion athe plane. Yesterday, I downloaded a lot of movies and youtube videos because when I return to Japan, I will fly for about 16 hours in total, so I don’t want to be bored on the plane.

In English, when we are talking about the youtube website itself, then we just say "youtube". If we're talking about the different videos we watch on youtube, then those are "youtube videos". I changed your sentence because you didn't mean to say that you downloaded the whole entire youtube website, just some videos. When you are talking about traveling by plane, boarding the plane, sitting inside and traveling in the plane, etc., we say "on a plane", rather than "in a plane". Same for bus, train, and boat. Examples: 1) You have to fly on a plane if you want to travel from America to Japan. 2) I rode on the bus to the next city. If you are specifically talking about being physically inside the plane, then you can use "in the plane" or "on the plane" Examples: 1) I put my luggage in the overhead bins in/on the plane. 2) The crying baby disturbed all the passengers in/on the plane. I changed your sentence to "on the plane" because it just sounded more natural to me in this case. I'm sorry it's hard to explain why :( Further reference: https://ell.stackexchange.com/questions/159151/which-preposition-is-correct-on-the-airplane-or-in-the-airplane https://www.englishforums.com/English/InOrOnThePlane/bgpkmq/post.htm

Yesterday, I downloaded a lot of movies andfroｍ youtube because when I return to Japan, I will fly for about 16 hours in total, so. I'm going back home to Japan and the flight takes 16 hours. I downloaded the movies because I don’'t want to be bored in a planeduring the flight. Yesterday, I downloaded a lot of movies froｍ youtube. I'm going back home to Japan and the flight takes 16 hours. I downloaded the movies because I don't want to be bored during the flight.

There are several ways to say this more naturally. Here's another way: Yesterday, I downloaded a lot of movies from Youtube. I'm flying back to Japan and the flight is 16 hours long. I don't want to be bored.

NetflixYoutube is very convenient. Youtube is very convenient.

This is confusing. At first, you said you downloaded movies from Youtube but now you're saying Netflix is very convenient. Whichever one it is, it should be the same in both places. To be congruent with the first mention of Youtube, this sentence should also refer to Youtube or change the reference to youtube at the beginning to Netflix.