I am at Santiago at present, working as a volunteer at the Pilgrim's Office, until 20 August. Peregrino Nicole is here as well, and we are currently flat-mates.
As regards the Holy Door, or the Door of Mercy, it is as everyone above has stated. I would add that, as this is the summer and it is peak pilgrim and tourist season, there is frequently a queue to enter the special Holy Door/Door of Mercy. When I walk by, there are typically 20-30 people in the queue. But, it is totally manageable.
The special door is on the East side of the Cathedral, facing Plaza Quitana. The primary entry to the Cathedral is around the corner, through the South Transept, at the top of the stairs at Plaza Praterias.
In either case, backpacks are NOT permitted into the Cathedral for security reasons. The nearest consigna, to pay to store your backpack, walking sticks, staff or bike securely is located at the top of the stone stairs at Plaza Praterias, about 30 meters to the right.
An organization of university academics operates a shop on the corner of the building called "Campus Stellae." They sell souvenirs, as well as locally sourced crafts and foods. But most important to this thread, it is the consigna nearest the Cathedral entrances.
The Pilgrim Office, at Rua das Carretas #33, also operates a consigna. It costs €2 per day. You CAN store items for multiple days, like if you want continue on to Fonistetre or Muxia, and return in 5 or 6 days. Inquire at the group office.
I hope this helps.