Dear You're weird~
I don't think it is you who is weird, but your stepfather and mother.
There are basically 2 ways to try to bring up kids, the first is to use love, friendship and have reasonable boundaries explained. Praise your achievements, encourage your goals and make you enjoy life and being with them and the rest of your family.
The other is to run a prison camp -your situation.
Its a fact, at 15 years you like all other teens will have different likes and dislikes to your parents and will do things they think are inadvisable. That's to be expected, it is normal.
True there are some things that go beyond this , drugs being an obvious one. Then the parent's problem is to stop the source and need, be it peer pressure, lack of cash, or being miserable without. Than could involve a search, but wiht you part of it and seeing the need for it. Cooperaton.
You do need privacy, anyone does, and you do need the chance to grow until you can face problems in life competently. Sometime that means parents have to stand back and watch those mistakes being made (a hard job to see someone you love endure hassles))
Your parents (yes I know only one is a real parent) are way beyond this into the area of cruelty. Explaining how to use a phone and social media is a good way to go, a blanket ban or removing the phone is counter-productive. How will you learn to deal wiht things if you have not met them?
Kids should be able to trust their parents, for love, for the necessities of life, advice and comfort when things go wrong.
Parents have to learn to trust their kids, if not the kids will grow up feeling they are untrustworthy, not good people.
Even if you do something very wrong (I"m not suggesting you have) then goal warden behavior is not the answer.
I'm sorry you are stuck in this situation, Sophie_M will try to help, and has given good links already.
I's also consider talking to at school , an adult you get on well with and might help.
How do your younger sisters get on? Do they have the same harsh treatment?