What are those yellow/brown-coloured texts with underlines and strikeouts in my document, accompanied by vertical lined on margins? And how do I get rid of those?
You have activated Track Changes mode ( ▸ ▸ ), e.g. using ⇧ Shift+Ctrl+C key shortcut. This is the mode when all modifications made into the document are recorded, and may be reviewed later. It is very useful when a document is created collaboratively, e.g. when you do rounds of edits, and need to send your edits to another party for review.
To turn the mode off, you need to disable recording the changes, and accept/reject the changes made so far.
Please note that in general, you should not just disable showing the changes (▸ ▸ or ▸ ), because this does not disable recording the changes, and while visually you would get uncluttered document, it will still contain the history of your edits. This could potentially pose you to the risk of exposing something to recipients that they weren't expected to see.