Hi!
Yes, you are correct. No reservations are needed to attend Mass or other liturgical services. There is a separate "Mass access only" queue, which is in front of the central portal, and is marked by a small white sign (please note the sign doesn't get put out until 30 minutes ahead of the Mass start time). Once you enter Notre Dame, you'll go directly to the Nave (the centre) to take a seat for Mass/the service. Entrance still subject to the available capacity inside the cathedral.
Entrance starts 30 minutes ahead of the start time of whichever Mass/service you're attending (except for Morning Mass/the first Mass of the day when entrance starts when Notre Dame opens, which is 7:50am on weekdays and at 8:15am on weekends). However, depending on which Sunday you attend, and which Mass, you may need to arrive earlier (ie. if you want to attend the 11:30am Mass, I recommend arriving about 45 minutes in advance).
If you attend Morning or Afternoon Mass, you can stay and visit the rest of the main floor of the cathedral without having to exit/re-enter with a time slot. FYI, Morning and Afternoon Mass during the week and on Saturday doesn't include the Notre Dame de Paris Choir and The Grand Organ is only played during the 10:00am, 11:30am and 6:00pm Mass on Sunday. Evening Mass runs right until closing, so there isn't time to visit the rest of the main floor of the cathedral after Evening Mass (except on Thursday when Notre Dame is open until 10:00pm).
Someone else already commented the link to my Reddit post with all the full details, that I do regularly keep updated (thanks, Lynn!), but the bell towers are currently still closed to visitors. The reopening date of the bell towers is still TBC, but as of now, it's estimated to be around mid-September. Advanced reservations will be required, and since the bell towers are considered a tourist site, there will be an entrance fee (this is no different from how it was before the fire). Tentatively, the entrance fee will be €16 per person.
(Also! Off-topic, but I just noticed that you're from Hershey. In what feels like a life time ago, I spent a lot of time there. I'm pretty sure I still hold the record for the most number of rides on the Skyrush rollercoaster at Hersheypark, haha)