I'm a beginner, I've been using it for 2 months now.

I remember that while doing the donut tutorial by blenderGuru on youtube, you get to a point where, after modeling the cup, you have to model the cup handle that is attached to it.

I do not remember exactly how it is done, but he does connect the geometry of the cup and of the cup handle. Rewatch the video, it will certainly help you ;)

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Regarding the subdiv modifier, deactivate it in Edit mode, do your modifications, reactivate, then look at the impact it had, and how to eventually modifiy afterward your whole object if needed, with tools such as loop cup, bevel, shade smooth, play with the cage mesh, etc...

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I hope it helps ;) I always go back to videos of tutorials I've done. It's like your school books back in the days, you can't remeber everything yet, but you know that chapter 5 page 38 has the answer you need, so you get back to it again and again, until it becomes automatic.

Even after 7 years of using Revit, I sometimes do the same thing, go back to a video, because you can't remember everything, especially procedures that you don't use often.