New Window Install

We are buying a new house in which we are gutting all the sheetrock down to the studs and will be getting new siding although not right away? We are replacing almost all the windows. A few questions.

1. Is it easire to install the windows while there is no sheetrock? We may get bigger windows( thus frame opening larger) so I guess while there is no sheetrock as that will have to be removed to frame anyway.

2. Is it easier to install windows during or when siding is removed? Really not planning on replacing siding right away but if it's a lot easier we may.

3. What is the thick white "trim" that I see sometime around the windows? Is it trim or capping and is it part of the siding install or window install? See the attachedc pic? Thanks

4. If I'm not doing a ddirect window replacment but making the opening larger for a bigger window I need what type window? One with flanges..obviously not a replacement window?? Thanks.

Also me and my friend have installed windows in my current home before with no issues( even reframing) but I'd imagine if I want to get designer type trim it becaoimes more difficult, correct?