Can members change my site and pagepost need protection?

Hey! Everyone. How are you?

I hope you all well, so you can answer my questions. Lol!

Actually this happened a few days ago. I have accidentally made my website accepting of members, with 1 click on general settings, that my website allows "any one 'to subscribe, even though I want them to subscribe to my email listings, but it's not running yet.

I was confused when I found out there was a strange name on my website user list and site support really helped me.

When I realized someone wanted to register as my member, I was very happy, but this made me have to find out about membership-based websites, or free membership plugins. [Lol! because this was not in my plans].

And thanks, I found blogs on WA that discusses website based membership. Then I had download+install+activies a membership plugin; Ultimate Member, [and I can't use the plugin suggested because it says: Untested with your version of WordPress].
Once I set it to protect page and posts, but it doesn't show up when I check from outside the site, so I changed it again to unprotected.

My current question is:

- Can the members change my website after logging in?

- Whether all posts and pages need to be protected. If it's not protected, can the members change my post / page?

- How about login page, register page, join us page, profile page, or the download page?

Well, I plan to give freebies/sell a bundle of something that my member can download it.

I'm still confused about protection, what page/posts needs to be protected? and what are the risks?

I appreciate any help and thank you so much in advance :)

Sylvia Yuchinta