Caty, it is my experience about
"ExtData folder - used by Garmin
GPS folder - stores ARC.BIN , BEE.BIN etc files used by webupdater"

ExtData folder - stores 006-D0952-01.bin, 006-D0952-02.bin, 006-D0952-03.bin, 006-D0952-04.bin, 006-D0952-05.bin, 006-D0952-06.bin, 006-D0952-07.bin, 006-D0952-08.bin files which contains special font for not alphabet languages likes Herbew, Thai, Arabic.
ExtData folder - also stores for some device wiht MTK chipset files ARC.BIN, BEE.BIN, 006-D1221-00.bin. 006-D1221-00.bin - GPS Chipset Type M HotFix Data software which updates by webupdater and ARC.BIN & BEE.BIN are files what stores current info for GPS software metioned above.

GPX folder have folder with archived tracks. It is named Archive My experience and tips for Nuvi only below:

1) In last software Garmin limit next -
only 20 files in this folder with current numbering, each file approximately 1Mb, in each file approximately 10000 trackpoint for 800-1000 km.
When current.gpx files reaches size mentioned above it is move to Archive folder with next current present number. current.gpx is clearing and write down over again. If your have 20 files with archived tracks then the file with the least number is erased and new file with new number in title is created.
(It is have logic. Old software made a one big file not limited in size. As result there was free memory overflow and so on fatal consequences for normal working. Apart from this huge file had a lot of mistakes and MS do not read it without manual tags edition.
2) Cos if you want have full archive of your tracks - make copy of archived tracks files occasionally. I have archive with 86 files from one of my Nuvi (~75 000 km).
3) Only track from current.gpx is shown in device. If you want to view current track as much as possible for example in offroad (limit as mentioned above 800-1000 км) you may clear current track by Nuvi menu. Remember what if you do not make copy then data from it will lose. I have occurrences then current track went to archive and I do not may to see my start part of way on winter large lake
4) Archived tracks may be clearing only on PC. So you need 20 Mb free memory on new device what reserved for its.