I can tell you what it is like from my own experience. Albeit things are a bit different now from pre-covid times.
The archives I go to is in a museum. We now have to wait at the outer door of the archives as an archives staff member now has to escort visitors in and out of the 'searchroom'.
Once in the archives room each visitor is given a locker to store their things; coat, bag etc. Pens are not allowed but you can use a pencil, and your own notepad/paper. I usually keep my Kindle with me too.
Each person has their own table and their own microfilm reader if this has been requested in advance. On the table is the items which have been pre-ordered before the visit. If you don't know how to use the microfilm reader the staff will show you how. Let the staff know if you are a first time visitor and they will no doubt guide you regarding where to find things.
I generally find that once people are settled into their own research a kind of excited hush descends and you almost feel oblivious to everyone else as you lose yourself in the past. I find that time seems to go very fast when you are in an archives so prioritise what you want to see.
I felt a little daunted about visiting the first time I went but I needn't have been as it not hard to pick things up and the staff are helpful.
I think if you wanted, you could let the staff know about your agoraphobia and discuss things which might make the visit better for you. Such as might it feel better for you to sit nearer to the door, etc?
I hope one day soon you will feel able to go. It is a very exciting and wonderful experience; do try to take the opportunity to go when you feel ready.
I wish you every success with this.
