Start page: https://www.gov.il/he/departments/general/gtfs_general_transit_feed_specifications
Includes a linked PDF that explains the formats in detail.
OSM wiki: https://wiki.openstreetmap.org/wiki/GTFS
GTFM standard reference: https://gtfs.org/schedule/reference/ (not really needed as the PDF explains everything)
Here's my understanding for the parts that are relevant for OSM import:
Firstly, files are downloaded via HTTPS in this link: https://gtfs.mot.gov.il/gtfsfiles/israel-public-transportation.zip