Difference between indexing a page in GSC and submitting a sitemap?

I do both but can anyone tell me what the difference is?

I presume when you index it’s just one page and submitting a sitemap you’re telling Google about your whole site.

I changed a page and got Google to reindex it but it’s not changed in the SERPs and in fact my position is dropping.

So I have resorted to submitting the xml sitemap in the hope this might work?