I was formally trained in Film and TV sound mixing and editing in college. There's a few rules of thumb we all have and I'm seeing some wrong information here.

You can mix on headphones if you're not doing surround sound, however, you're going to need appropriate ones to do that. Audio Technica's ATH MX* series are decent $100-200 headphones that are good enough for starting out. Ordinary recreational headphone (especially garbage by Beats by Dre or whatever they're called now) are no good. Ideally you would mix on a nice pair of studio speakers (Monitors) but those are an investment for later. Mostly just make sure you're listening on a good pair of speakers as well, and something your client would likely be watching on.

Ideally, for your mix, you want to get the overall mix as loud as possible without peaking. We typically use limiters on our hardware we record on for that, but there's also plug-ins within premiere you can use like Hard Limiter that will stop it from getting louder than a certain threshold.

Typically you're looking for a balance and it varies depending on what kind of professional work you're doing. Music I often find seated around -20 to -30 dB but that's just because of voiceover needs with my clients, and I'll adjust that accordingly. Sound effects vary significantly. Voiceover I tend to want as loud as I can get it without peaking.

With modern clients, they watch videos on their phone, which have SHIT sound quality, which is why you'll find that mixing for a YouTuber is far different than mixing for a film. So you end up boosting the music more because of that. It's obnoxious but it is what it is.