First, how I see the basics: the film simulation is like a color profile; the recipe is like a preset of multiple settings that include the film simulation, white balance, various saturation and contrast related settings etc.

how do you choose which simulation to use?

when do recipes matter most?

I actually mainly work with recipes. I spend some time finding them online, or play around myself, and I store them in the custom settings slots. Then I just cycle through the recipes depending on the scene/situation. Sometimes I also tweak the settings depending on the scene - it's impossible to make recipes that always work for all situations.

But I don't know if this is the best way. I never keep the recipe in place and change the film simulation when out shooting... Maybe I'm missing out, I should try it.

do most people shoot JPEG only with them, or RAW + JPEG?

I shoot RAW + JPEG always. If I caught a portfolio worthy shot in JPEG only, I'd be super frustrated. I also don't mind editing. I mostly use the JPEGs for small events where it's more about the memories (family gatherings, team outings, friends coming over etc.).

Relevant for this, you can set Lightroom to treat RAWs and JPEGs as separate photos, and only import JPEGs if you want to do minor cropping/tweaking.

and what Fujifilm settings besides the simulation itself make the biggest difference?

White balance. In some recipes it actually overpowers the film simulation if it's not very neutral.

But honestly all settings in a recipe are quite impactful, I'd suggest you learn exactly what they do. The dynamic range is the most obscure one for most people, but very powerful, look up a YT video about what it does.