Baby led weaning

I’m not really sure where to post this so I’ll probably post in multiple groups.

My babies only 3 months so I still have a couple months left. But I’ve been looking into baby led weaning for when he can start eating solids and I’m a little confused. They say you should stick to one food for about a week when introducing solids, is that only for purées or does that also include BLW? From what I’ve read it seems as though you just place multiple foods in front of baby and let them do their thing. How would you determine causes of allergies if they’re given multiple foods at once?