If you run a YouTube channel approved for monetisation, one of the ad formats available to you is 'automatic mid roll ads'. YouTube explains that these are ads "shown in the middle of the video ('mid-roll'), not just at the beginning".
The easiest way to see that you have this option is by going to your 'upload defaults' settings page at https://www.youtube.com/upload_defaults, which will let you enable or disable automatic mid roll ads as the default for future uploads:
You can of course override your default settings on individual videos. Perhaps you have enabled automatic mid roll ads generally, but want them turned off for one particular video. Or conversely you might not be making use of automatic mid roll ads but want to try them out on just one video.
However when you edit a YouTube video and go to the 'monetisation' settings, you won't see 'automatic mid roll ads' listed as an option anywhere. Why not? Where has it gone?
The first thing to note is that automatic mid roll ads can only be used on videos that are at least 10 minutes long - what YouTube refers to as 'long form videos'. But even if your video is more than 10 minutes long, you won't see 'automatic mid roll ads' in the individual video monetisation settings, because YouTube lumps them in under 'ad breaks' (along with before-video and after-video ads). You'll find the 'ad breaks' options at the bottom of the settings page, under 'ad formats':
From this point on, the information on YouTube Help and my own personal experience part ways somewhat. The YouTube Help instructions for setting mid-roll breaks on one video are simply: "Beneath 'Location of video ads', tick the box next to 'During video'.
When I edit a video, however, there is no 'Location of video ads' section and no tickbox that says 'During video'. Instead I can click a button to get YouTube to 'Suggest placements'. The tooltip here explains:
YouTube can suggest where to place ads based on the content of your video.
Video must be public and in-stream enabled for suggestions.
Note that using 'Suggest placements' will remove any ad breaks that you have manually added (though YouTube helpfully warns you that this is the case). Conversely, inserting your own ad break will clear off any suggested ad break placements. It's an either/or situation. And if you do select 'Suggest placements', don't forget to hit 'Save'.
In my experience, 'Suggest placements' seems fairly conservative, only suggesting one or two breaks in the average 10-minute video. And the suggestions can be removed - either individually by clicking on one then clicking on the 'X', or in bulk by clicking on the 'Edit ad breaks in bulk' button.
One other thing - it isn't possible to edit mid-roll ads through the YouTube Studio app (at least the iOS app, and I am assuming the Android app is similar). The app helpfully tells you: "To edit mid-roll ads, visit YouTube on your computer".