I am a birder who became interested in every species I see on trails.

Some of the resources I find helpful when creating and identifying observations:
PLANTS:
http://www.sdplantatlas.org/ (iNat Photo Guide for Plants is under "Misc Info" tab)
http://sdplantatlas.org/pdffiles/SDCoChecklist5ed2014.pdf (Checklist of San Diego Plants)
Checklist of Vascular Plants of Mission Trails Regional Park
https://borregowildflowers.com/index.html (Desert wildflowers)
https://dn.com.au/Dean_Nicolle_galleries.html (Eucalyptae)
https://forum.inaturalist.org/t/how-to-identify-grasses/8538 (Grasses)
BIRDS:
http://www.sdplantatlas.org/BirdAtlas/BirdPages.aspx
REPTILES AND AMPHIBIANS:
http://herpatlas.sdnhm.org/
MAMMALS:
San Diego County Mammal Atlas (sold in SDNHM Gift Shop)
MOLLUSKS:
https://www.inaturalist.org/people/50920 (Susan Hewitt's photo guidelines)
INSECTS:
https://bugguide.net/node/view/15740
https://sites.google.com/view/flyguide (Diptera)
https://cjai.biologicalsurvey.ca/mw_41/mw_41.html (Ambush Bugs)
GALLS
https://www.inaturalist.org/projects/california-plants-with-mystery-galls/journal/39523-130-california-galls-listed-by-plant
GENERAL:
http://nathistoc.bio.uci.edu/ (Natural History of Orange County)
https://forum.inaturalist.org/t/identification-etiquette-on-inaturalist-wiki/1503 (ID Tips)
https://www.sdnhm.org/education/community-science/ (has link to "Quick Guide" pdf)
https://forum.inaturalist.org/t/identification-quality-on-inaturalist/7507 (ID Quality Study)
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki/63 (Search URLs on iNat)