I came across this gem online that someone wrote up on how to use the API. I thought it was one of the best resources I’ve found yet that is not requiring any paid service. I hope that some newer people who want to learn how to use the API can find this helpful. These articles have been posted recently as June 2021.
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-1-introduction/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-2-variables-declaration-and-assignment/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-3-understanding-the-solidworks-object-model-part-1/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-4-understanding-the-solidworks-object-model-part-2/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-5-understanding-the-solidworks-object-model-part-3/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-6-understanding-the-solidworks-object-model-part-4/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-7-user-declared-functions-part-1/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-7-user-declared-functions-part-2/
https://wiki.cadcam.com.my/knowledgebase/solidworks-api-fundamentals-9-classes-part-1/