This is ‘edit’ mode. Click the little button in the top right, to the left of the ‘x’ to move between ‘edit’ and ‘preview’ mode. Now, you might find this a bit annoying, however, there’s a good reason for this. When you set up Obsidian you choose a place for all your notes to go (either locally on your computer or in a cloud folder like google drive, dropbox etc). They will be saved there in the most universal format in all of computing: as text files with ‘markdown’ formatting, and images. Barring some very unlikely change in computing culture, these are just about the most ‘future proof’ formats you can get, and so even if ‘Obsidian’ closes down or becomes obsolete, there are a gajillion other programs that use the same format. The problem with other purely cloud-based tools like ‘Evernote’ is that all your stuff is all saved on their servers in their special format and if they close down you are a bit screwed .
This is one of the reasons I recommend Obsidian specifically for literature reviews. This is the kind of knowledge you may want to have access to for your whole career (even more so for the literature reviews you will do on your PhD if you do one). I don’t think Obsidian is quite as good for lecture notes as it is for paper notes (for lecture notes I prefer ‘Outliner’ style programs that allow you to make nested bullet points and have more formatting options, like Dynalist or Workflowy), but if you have lecture notes you think you want to keep forever, it might be worth thinking about keeping them in Obsidian too.
When you start adding more stuff from the paper you want to be able to organise, and we can use ‘headings’ for this. Type # then a space then type your heading. For example you might want to separate content from the Introduction, Method, Results and Discussion sections of the paper you’re reading. For example, below you can see that in the ‘preview’ mode these become proper headings – if you go into settings and find the right option, you can even get the option to collapse them to navigate your note more easily. A single # makes the highest-level heading – if you want subheadings you can use ## and even ###. Just make sure for headings you put a space after the hashtag, otherwise you will make a tag, which we’re going to talk about next. Below you can see this in both edit mode and then preview mode.
 I wouldn’t bank on Evernote staying around forever, they are notoriously badly managed. They also have unreliable customer service in my experience so I wouldn’t trust them to provide a good ‘transfer’ service of your data if they did shut down. If you can’t tell, I used to use Evernote, but switched, for exactly this reason. I was slowly building up a bigger and bigger library of notes on important papers to me, and the prospect of losing them all became too much.