How can I repair damaged plaster when I come to repaint?

I have a couple of problems with the plaster of my house, and when it comes to repainting, I want to try to fix them.

I’ve watched some youtube videos, and done a lot of googling, and although my search has produced a variety of products that I can use, I’ve had conflicting information and also such a wide range of products, that I am at a loss of how to proceed. It’s also quite hard to find exact details of procedures, as there are many topics on what to use, but it’s unclear how to apply.

photographs of problem areas are below.

Some are caused by penetrating damp, which is now fixed.

For the green, my plan was to try to sand the bumps away, and then apply something like "Polycell Ready Mixed Plaster Repair".

It’s unclear if I have to remove the paint first, or how to even remove the paint as it’s not really flaking. I was basically going to slap some on, and buy one of those trowels and try to smooth it as flat as possible, then when it is dry, sand it and paint it (first adding diluted paint as a primer).

For the yellow, I am really not sure, perhaps I can do the same thing?

The white at the bottom I really have no idea, I suppose again, the same thing but I am not sure how to get a clean edge.

I have read things about pva glue, stain block, joint compound (?) and all kinds of stuff, so I am overwhelmed. I am a total DIY beginner, the best I’ve done so far is just sanding and painting.

Any advice would be welcome.