IOPs are all different. I went through two, one I flunked out of and one that saved my life. Neither was 12-step based. The first one catered mostly to down-and-outs who had gone through all kinds of experiences (many associated with meth and heroin) that I couldn't relate to, and did push outside participation in 12-step programs but didn't require it. The second one catered more to middle-age alcohol addicts who I could relate to, and didn't push 12-step at all (though a couple of the counselors individually did). Other IOPs are heavily based around 12-step programs, so it pays to shop around if that's not what you are looking for (it sure wasn't what I was looking for).
I never did inpatient. The first IOP wanted to send me to one, but I didn't really want to go and managed to avoid it by continuing to drink (they required 3 days sobriety before they'd admit you, since they did not have detox facilities). But from what I hear from other folks, it's the same story, pays to shop around.