How can I keep my Edge loops more smooth?

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Sorry if the title is confusing
This topic is related to my other topic, “How do I merge these 2 loops?”
Whenever I want to delete the vertices to create an edge loop, so I can merge the 2 objects, they curve up and look weird. I made sure they have the same amount of vertices on both objects.
How do I make the edge loops look more smooth and round?
And I know I’m repeating this question again, but once the edge loops are smooth, how do I merge them? can someone give me a step by step?
If you’re going to reply, please be nice to me. I am new to blender and would really like to be respected and not yelled at just because I don’t know that much

This tutorial is pretty old but still good.

At around 07:50 he merges the upper and lower torso.

Basically, select the two opposite edge(s) and bridge them.
bridge

It looks little bit confusing as you have subdivision modifier turned on. I would recommend you to turn it off while modeling. You have few options how to connect it. You can select edge loops and then bridge them or manually select two edges and press F it will fill the space between them with a polygon. Also both object hast to be one. If you modeled them separatedly, then you have to join them together.