Unless specified otherwise (such as a Creative Commons license), all of the videos would require permission to use further.
You will need to gain permission from the other YouTube videos' creators. This type of video is considered a compilation and there is a very fine line for using compiled videos under fair use.
Youtube provides this program where if your videos get a lot activity (views) then they will pay you to showcase ad's on your video. Your videos need to not have any copyrighted stuff in it, as in music and so forth. You can look this up in Youtube's help section.
You would need to visit a website which contains Creative commons licensed content. Some YouTube videos are under Creative Commons though. You can find them by enabling the "CC" filter in YouTube Search.
You don't need permission from the company to do "Let's Play" videos. In most cases, the use of copyrighted materials would be acceptable under the fair use exceptions.
You would need the video file. Uploading copyrighted content without permission is not allowed and may result in the removal of your video.
You would need to use a movie editing software. Some music that is copyrighted will be removed from YouTube, so make sure you have permission before uploading any copyrighted content.
To use others' copyrighted works, you need an exemption in the law (such as fair use) or permission from the copyright holder.
You would need to gain permission to use copyrighted content if you are going to make money from it. Generally, you can use copyrighted content (covering songs and such) if you make no money from it. The copyright holders might put adverts on your video so that they make money from it instead.
You will need to search the internet for 'non copyrighted videos'.
No. An account on youtube is only required for: uploading videos, commenting on videos, rating videos, etc.
Yes, your videos are viewable if you upload them on YouTube. First however you need to create an account, with YouTube.
Not to browse videos but if you wish to upload your own videos, comment on videos or like, dislike etc you need an account