There's a lot to do.
With Rachel & Kim, you usually can only talk with them at your house or theirs. Fight/Gather resources with them at each location (sand, sky, forest). When they show online, I usually go to their homes and invite them there. Then you can do more than talk. Sometimes, after they go offline after such an event, you can click the invite and they will show back up. I did this with Jennie like 5 times in a row once. It was beautiful. Got through a lot of content. Later on there is content that both of them need to be online at the same time, and yes, sometimes that invite trick works for that too.
For Dave's quest, you need to see when Jennie is online (early morning or late night, not always, 30% chance?), invite her to her house, and talk to her there about the quest. Also, she can clone Rachel and Kim too. To advance with Jennie, you need to go through all the scenes with her being different people to get her stats up higher. Eventually you'll be able to go on a date with her IRL.
Regarding the Dev Tool. On the PC in your room is a hack tool minigame. You need to get up to level 3 at least, then when fighting enemies you can hack them (they need to be low on health and a weird multi-colored circle with shapes will appear on their person during the fight, click that), then you can have sex scenes with them. If you invite any of the girls to your house, you can use the dev tool on them and it'll "freeze" their avatar and you can have your way with it. You also need to be able to use this for Dave's quest. (The higher your hacking level, the easier it is to hack an opponent.)
I don't think you need to fight/gather resources to advance the game, but you do need to interact with the others in the VR realm. Some IRL content is locked behind the VR content.
Hope this helps.
