My lo is 15 months and when you change her you lay the diaper on the floor she walks over and sits on the diaper is she ready for potty training?

Yes. Behvioral signs like what your mention may be the start to toilet training. Demonstrating a desire and showing interest are positive signs. If he broadcasts his bodily functions-announcing pee and poop and bowel movements are predictable and less wet diapers are apparent-you are on your way. Despising wet and dirty diapers helps.
Ready to try: Get a potty and see what she does with it - readiness for training is highly individual.
Doubt it. Data confirm only 25% are potty trained at age 2 increasing to 75% at 3. I see nothing in the observation you provided to suggest anything more than she understands she is having her diaper changed.
Not until she can. Walk from room to room, pull her pants up & down & say " pee-pee", "poo-poo" or other words to indicate she senses the urge to "go." You can model using the toilet & let her sit on a potty chair if she wishes. She has learned the diaper-changing routine & seems compliant, so she can learn to drink from an open-mouth cup, feed herself with a spoon, point to 3 body parts & 3 pictures in a book.