So, you're meant to get exp after every video you make, which is why it looks like 1 per day, but also get a bigger amount for the following:
- First time you have sex with a new character
- Any event in which you have sex (not videos, but story events - a smaller amount compared to your first time with them)
- During major events in which you could theoretically be thought to, as a character, have grown in experience or wisdom, etc.
As you say, some of it isn't fleshed out at the moment, and I'm planning more ways - such as the gym when it's opened being a way to farm it, and you getting more for doing other business events such as corporate events, sponsorships, and sex work, etc.