Talking on the YouTube front, a trivially small donation will support them far more than watching ads ever could. Even something as small as $1/year is often far more than they would ever make from you in a year. As far as donations to developers go, it depends entirely on what you feel comfortable with. Most people who work on open source projects are unpaid volunteers, so it isn't expected that you donate, but if you choose to do so it can be quite helpful to sustaining the project. If many people in the userbase were to make small donations, that would go a long way.
In reality, ads almost entirely benefit exploitative multi-billion dollar companies such as Google and Facebook, so my personal philosophy stands against them. I much prefer donating to people directly to cut out the exploitative middle-man.