Try to search for the site via keyword search in a search engine but the topic needs to be worthy of looking up of course.

Well, most of the time I ignore it, but if the topic is interesting I might use the url if it's shortened url, or search using google if not mmm...

Yes, if the topic interest me.

Unless the subject really unknown to me and interests me, I ignore it and keep reading.
As for clicking the links, no I never click any link blindly. I rather try to search for the site via keyword search in a search engine.
Though some sites like Wikipedia or Wikia are trusted. In that case, you type the domain name and rest of the url yourself anyway.
Just remember to check the text well if you go by QR code. May use site(s) like Virustotal to check it, if you're in doubt. And keep your website 'permissions' in check. Also use Ad-Blockers, multiple ones if your system can handle them easily.

Yes, I find that this is a huge problem with the scanlation medium. There is simply no way to link to external resources from translator notes. When the subject is too complex to be fully described in a note, and yet when you really care that the readers familiarize themselves with it in order to understand the text in the manga, there is a huge temptation to link its description somewhere on the page. I especially like to linking to Youtube videos for songs that a character might sing. However, the chances of someone retyping those often complex URLs are between slim and none. It is somewhat frustrating.

I don't think I've ever seen links in manga. I probably would ignore it since it's a pain in the ass to type it out, but I do click links I find in LN translations tho.