help and informations about a website (online store)

Hello guys
I am trying to make a website (an online store), but I have some issues. I am using siteground for hosting (growbig sub) and elementor pro for building it. I want to make it fully custom, but I have many questions and issues about basic things.
For the theme, I am using oceanWP; for now, maybe ill switch to Astra.
Some of my issues:
First, I created a template for the header. I put it on the theme customize section, and when I open it, it is fine, but after a refresh, I see this gap.
I don't know why maybe cause there is some mistake with the padding or margin or some settings with the top bar-tittle on editing the template or some problem with elementor and theme or a plugin issue (or maybe cause I put custom font)
It's weird cause the first time I load the site, it's fine, or when I am pressing ctrl+f5, it's working fine, but after some refresh of the page its shows that gap.
other things I want to learn, get some help with:
Any tip on making the site faster or keeping it that way for the future maybe something I need to give special focus from the beginning.
Any information on how to update pages and making them fully synchronized correctly.
Any tip on themes (as I said, I am using oceanwp, for now, is there any other that works better with elementor, woocommerce, or it's faster, in your opinion.
Any plugins or useful tools you would suggest.

Any help would be nice and much appreciated. Have a nice day

(sorry for any mistake or misunderstanding, my English isn't that good)

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I don't know how to edit it but for the attachments, I add them cause I wanted to say that the transparent effect works differently on some pages maybe its an issue with the scrolling effect on each page ( I think you can choose between scrolling and fixed or something I would say that's the problem but I am not sure I am looking at it) I forgot to remove them so don't pay attention to them.