In a pan. ;) General rule of thumb -- heat the pan first, then add the fat and/or whatever you plan to cook. Works for just about everything. For meat, I always start hot, and reduce the heat as things begin cooking. Ground beef can take a good amount of heat, as long as you keep it moving -- if you're in the middle of other prep work, or if you're at risk of other distractions, turn it down -- cooking it slow won't hurt it. It'll just be slower. Bottom line is you can always cook food more -- but you can't uncook it if it's overcooked or, worse, burnt.

I'd encourage you not to overthink this process. It's browing meat. It doesn't get much easier. Turn on the heat. Throw it in the pan. After a time or two, you'll figure out what works for you. You will have to try really hard to under- or over-cook it.

Varies by cut of meat (see the very similar thread going on now.)

If fatty, slow-cook with bone broth or fry in its own fat at medium-to-low heat.

If very lean, slow-cook with bone-broth or fry a couple pieces of bacon and cook the grassfed beef in the bacon fat.

A very hot pan, do not crowd the pan, and brown it fast. Do in batches if pan isnt large enough. After that i transfer the meat to dutch oven and simmer it slowly with stock and other stuff, vegs and spices.