Actually, just an analog mic preamp (and for the SM7B or other very low output mics: one with super low noise or a cloudlifter in between) or compact mixer will get you up to line level, and then you're good to go with a motherboard sound interface if it has a line input jack.
A better ADC (like in virtually any outboard audio interface) certainly doesn't hurt and would definitely be advisable for musicians, but it's far from necessary for a youtuber / home office use case.
My experience is that a high-end device like MixPre II is fine with a dynamic mic. Focusrite or Tascam US needs a cloudlifter to get enough (clean) gain. Random stuff like BlueYeti is hopeless with crackling noise.
So maybe you don't need a great ADC but you have to choose your audio interface very carefully (for its preamp.)
Yes, the preamp quality is more crucial than the ADC quality when it comes to using mics with very low output, unless using a cloudlifter which shims away this issue. Way more likely to be dissatisfied due to preamp noise than due to a cheap ADC.
Typical interfaces have a fine ADC and some kind of pre, but the pre might be noisier than desired when paired with an unusually low-output microphone, so if you already have an ADC via motherboard line input then you "just" need a pre. I don't mean to lump all dynamic mics into this problematic low-output category (SM58 is much higher output than SM7B, both are dynamic) but we could certainly say that condenser mics should never be in this category when a few inches from someone's face.
It sounds like you know more about this stuff than I do - I’ve spent hours searching online but it’s hard to find good info.
Why are mics like the SM7b better than mics like the MV7 or elgato one that just work over usb? I don’t understand how they actually work or why all the extra equipment (audio interface, gain booster, cables) is worth it - what’s the tradeoff being made when you use USB?
All mics sound different. Whether or not the mic includes a USB interface doesn't inherently make things worse. It just happens to be that the best sounding mics rarely come with an integrated USB interface. No need to use the best if you're happy with something else.
It's like asking what makes a VM better for a website when I could just use a SaaS site builder. Use whatever works; depending on your goals you might end up with the exact same finished product.
The SM7B is just trendy because a lot of famous podcasters use one. For just recording dialog, or doing video calls something simpler would be just fine.
Getting super high end with microphones is only really applicable in the music industry IMO.
Fun fact: Michael Jackson recorded the vocals for Thriller on an SM7B.
If you want the Shure SM7B you need an audio interface (and probably also a cloud lift or dynamite to bump up the gain).
Lots of streamers, podcasters, youtube people use them.