I have never canned. My grandmother did. I know if done wrong you could die from botulism. When I open canned goods I always make sure it sucks air in. Do not eat it if the vacuum has been lost. Never buy dented canned goods because the dent forms a weak spot that could make it leak the dent. Here you can download a lot of books for free about canning.
https://b-ok.cc/s/canning