0 = Lace [fingering 10 count crochet thread or light fingering yarn or cobweb w eight] — Steel hook 6, 7, 8 or B-1 hook
1 = Super Fine [sock, fingering baby yarn] — B-1 to E-4 hook
2 = Fine [sport, baby yarn] — E-4 to 7 hook
3 = Light [DK {double knit}, light worsted yarn] — 7 to I-9 hook
4 = Medium [worsted, afghan, aran yarn] — I-9 to K-10 1/2 hook
5 = Bulk [chunky, craft, rug yarn] — K-10 1/2 to M-13 hook
6 = Super Bulky [super bulky, roving yarn] — M-13 to Q hook
7 = Jumbo [jumbo, roving yarn] — Q and larger
Specialty Yarns and Fabrics {stripped for crochet Yarn Weights

https://sarahmaker.com/yarn-weights-guide/

https://www.nickishomemadecrafts.com/crochet-perfect-straight-seam-rounds-photo-video-tutorial/

https://www.meladorascreations.com/invisible-seam-free-crochet-pattern/

https://oombawkadesigncrochet.com/2014/02/how-to-join-new-yarn-with-single.html#_a5y_p=3973748

Has video and written instructions.

http://crochetmelovely.tumblr.com/post/102543222390/veryhelpful

https://www.mariasbluecrayon.com/2017/05/crochet-tipget-evenrows-without.html

Each yarn package will have a recommended crochet hook size to be used with it, depending on the yarn weight.  Yarn weight has very little to do with the actual weight (heaviness), it is more to do with the thickness of a yarn strand. Most yarn manufacturers use a common standard of numbering the yarn weight so that a crocheter or knitter can easily substitute the yarn whenever needed.

https://www.knitpro.eu/blog/what-crochet-hook-to-use-with-what-yarn/en