Montessori at 18 months is very different from a Montessori preschool, at least if it’s anything like the schools we’ve gone to.
Between 18 months and 3 years, the program is very much focused on play, sensory experience, and social skills. It’s a program specifically designed for that age group and is for the most part a lot of fun for the children - while helping them learn basic skills that will help them have something to build on in the future.
Montessori is very much focused on the whole person, at least when fully following the curriculum of the AMI (or similar organizations); even in older ages the focus is not as much on academics as other philosophies. They certainly do well academically, but much of the day also involves learning basic life skills like cooking, cleaning, self care, and other things that are often not as much of a focus in other areas.
The typical program is not actually a full day; most seem to go until 11:30 and then after that they may offer daycare for the remainder of the day for children whose parents need the care.
Some links:
Cambridge Montessori Toddler Curriculum - goes into some detail about the different areas and how they are helpful for future learning. Note that a lot of the things they do are similar to what you might find in a non-Montessori environment - just a bit more organized.
NAMTA explains the different varieties of pre-primary class; your child would be in the Young Children's Community, and it explains how the classroom is laid out to help the child explore and develop.
NAMC has a lot of resources discussing the Montessori philosophy relating to toddlers, including teaching guides, which will be used by NAMC-accredited centers (AMI, NAMC, etc. are different accreditation organizations, and while broadly similar will have differences.)