The most important thing to understand about modern car insurance prices is that the BIGGEST influence on your rate is YOU. Your credit score, previous insurance history, tickets and accidents, age, marriage status, other people in the household who might have access to the car and their credit score etc, it all has a larger effect on your rating than anything else when it comes to typical cars. That's all you need to know. I have folks who pay $150/mo for a household of brand new luxury cars with great coverage, and people who pay over $500/mo for ONE car for minimal coverage.
Allow me to preface modifications with the fact that if any of your modifications violate the law, and you get into an accident where you are partly liable, the insurance company may speculate that the modification caused the accident and deny the claim. 12% of all bodily injury lawsuits resulting from car accidents in 2018 were over $200,000, which would be your bill if you are denied a liability claim so I would go to extra lengths to stay legal.
Regarding your modifications, this affects your collision and comprehensive property rating group classification. If you have more stuff on your car that is more valuable than the factory parts, it would cost more to repair or replace those parts, so it will cost more to insure the car... substantially more per dollar of worth over the stock worth of the car. If you do not report the modifications on your car, it's not a big deal. you'll just have to foot the bill yourself for re-modifying the car. Generally speaking, just like doing your own work on your own car, it costs less to insure the car yourself by always having 50% to 100% of the value of the car and its modifications brand new in a bank account or in some way accumulating interest, rather than insuring it continuously over a long period of time.
Having too much debt and too much insurance will make you poorer in the long run, hurting your credit, and raising the cost of insurance and financial products, and it goes in a circle. You won't get in trouble NOT reporting the modifications as long as they wouldn't otherwise affect the safety of the vehicle or make the vehicle illegal to operate on the road.
source: I'm an insurance broker