I'm such a noob I don't even know how to record the speedrun. could someone help?

For PC, I use OBS, pretty easy to use and works very well in my experience.

For Console, unless you have a capture card, recording your TV or handheld with your phone works.

For Mobile, iOS has a built in screen recorder, I believe you can download some for Android.

For android you can use mobizen or movavi If youre concerned about the quality of the video, you just need to make sure that the gameplay is visible and that the quality is good enough to see what youre doing in the game and the same applies to the framerrate. Audio isnt required for many boards but it might be required for top level runs.