I live in an area with a very large Indian, especially Punjabi, population. Many of my friends are Indian and they've shared their recipes with me. There are many Indian grocery stores here, so it's easy to get all the spices, mustard oil, pickles, Indian vegetables, etc.
I make my own style of garam masala for basic curry sauces, and change it up as necessary. I use whole coriander, cumin, black cardamom, cinnamon, cloves, nutmeg, mace, toast the seeds, grind them and it's called Garam Masala. Garam actually means "Warm", and masala is a mix. GM isn't hot, it's warm and aromatic. Heat in the dish comes from added chilies.
There are as many types of GM as there are families in Indian, but the important thing is that GM must be tempered in oil.
Tempering is the process of heating the gound spices in hot oil or ghee before the mixture is added to a dish
You could also temper a small amount of the spices to add at the end of cooking. My Indian friend called that a Tarka, it's a mix with onions etc., and she makes and freezes it in batches. Very flavourful. It is also called Tadka.
I'll add other spices to the dish along with the GM, such as fenugreek seeds, amchur (powdered dry mango), tumeric, kashmiri chili, cardamom, asafoetida, etc., etc.
indian dishes I've made lately are Chickpea curry (Chana masala), Chicken curry, biriyani with chicken, Halibut curry - basically, curry is a sauce for anything.
Butter Chicken isn't actually Indian, you can use thighs or whatever you want. People usually prefer thighs because they don't dry out as quickly.
Indian cooks keep the chicken on the bone, and usually, remove the bones/skin after cooking.