To give a more detailed answer, as someone who attended a Montessori school from preschool through 8th grade that was embedded in a public school and had class sizes of about ~30, the biggest differences from my student perspective compared to the traditional high school I attended afterwards:

1. Individual autonomy of the student in time management. To give an example, in 1st through 5th grade I wrote up a new contract at the beginning of the week with my teacher with a list of goals to accomplish by the end of the week, and then maybe half the day was pre-scheduled lessons from the teacher (some whole-class, some mini group, some individual), and then you could choose when you'd work on each lesson/goal at any given time. If you didn't want to do your math lesson until 3pm you didn't have to. You could do reading first then math later. Or vice versa. So in a sense you're learning time management and planning at a much earlier age and you're given freedom to make mistakes there, learn the perils of procrastination (or fail to learn them and develop bad habits from an early age which I saw to be quite common among my peers), and you feel like you're in less of a prison (after having autonomy up until 9th grade when I left Montessori you have no idea how much highschool felt like prison).

In middle school (6th through 8th) they took that a step further, and you might be given a list of objectives that needed to be done in a number of weeks or even in a quarter, and it was up to you to spend the 8 hours a day effectively to accomplish them by the end. It was therefore quite common for students to slack and just chat with each other for large portions of time only to crunch at the end. You could theoretically have hardly any homework if you just worked all day at school or you could slack for weeks on weeks then work all night very night and weekends for a few weeks at the end.

2. Student autonomy of location and freedom of movement. You were not forced to sit at a desk all day, you could move around the large daycare-like classroom, sit on the floor with students over there, or work at a table over here, or use an abacus over there. In elementary there was an emphasis on hands on learning. Learning the letters by having the teacher guide you to trace them in sand. Learning multiplication and division using physical beads and blocks to give you an understanding of multiplication with a geometric, spatial analogy. So you might hae a lesson scheduled to play 'the banker's game' where the teacher would take you over to some blocks and have you add, subtract, multiply, or do division using beads (https://www.youtube.com/watch?v=ysIvxeErRp0)

There was less of that physical grounding in later elementary and practically none in middle school but you were free to move around and be wherever you wanted to be in the classroom, with whichever students you wanted to be with, or be alone. And to a certain extent you were allowed to waste time or socialize as well.

3. At times, freedom to determine the medium through which you would present or demonstrate your knowledge, and freedom to choose topics or subjects to study to a greater extent and at a much earlier age than is common in normal school. Montessori seems to place more emphasis on student-curiosity as the mechanism for choosing project subjects, medium of presentation (it's quite common for you to be able to choose to "learn" and then present some social studies subject through an essay, or a powerpoint, or a poster board, or a speech, or maybe even through a play.

4. Emphasis on student government and Socractic processes. My middle school program had us collectively run a community meeting for the class for about 20-60 minutes each day, the moderation/facilitation of the meeting rotated each week to a new group of 3-5 students, you had a portion of the meeting for acknowledgements (acknowledging someone else's accomplishment in the class), problem solving (raising issues or problems affecting the class community), which led into proposals (suggesting a proposal of a rule change, an action to be taken, etc. to solve the "problem" - example - purchase lockboxes with student funds to store scissors since people kept stealing/losing them -, then argument and debate, then proposal of any amendments or tabling the discussion, then a voting section to vote for, against, or abstain for those proposals. It was kind of a parliamentary system which was a little more complicated than that but that's the basic idea. And we had a lot of Socratic discussions as well, usually at the end of a meeting (https://en.wikipedia.org/wiki/Socratic_questioning). Of and we had to write a class constitution via a constitutional convention and then our meetings followed that constitution.

5. Overall experimental learning methods. An example would be that in our 7th-8th combined classroom, all of the freedom meant that one quarter our class was being extremely lazy and not doing work so we were missing deadlines. The teachers decided to take the students who had missed deadlines and named them the 'structured' group and put them in one room where speaking was no longer allowed without raising your hand. And the students who didn't have any missed deadlines were allowed to be in the other room and still chit chat, freely speak with each other, and be mostly autonomous. Once you got caught up again and didn't have any past due projects, you could leave the 'structured' no talking room and rejoin the rest of the class. This motivated a lot of students who'd been slacking to get caught up. Of course, there were some who were stuck in the quiet room for weeks.

The funniest part is that as barbaric and prison-like that 'silent' room seemed at the time to us, it turned out that normal highschool turned out to be almost exactly like that 'torture' room. Go figure.

6. Last, inequality of educational outcome. Many, many of my peers were exemplary students and the freedom allowed them to pursue their passions, become excellent students who were well beyond the normal curriculum in a specific subject that they were passionate about, and were excellent writers, speakers, collaborators, and had great time management skills. But other students took the freedom to chit chat and not work, hippy grading system, and other freedoms and just didn't do anything or wasted time. Many students in particular at my school district got a very poor grounding in math. So your mileage my vary.