ansdesigns

Hello Fabricio
Save As…  will let you choose a new file name and destination to save to. But the file type will only be readable by one of Affinity’s apps, currently; Affinity Photo or Designer.
All layers, filters, adjustments etc… will be retained and can be worked with again should you wish too at a later date.

Export…  will let you export a flattened final image that can be opened in other image viewers/editors, in the usual file types; e.g. jpeg, png, tiff, etc…
In addition to these image formats you can also export in Photoshop’s .psd format.
Like Affinity’s own format, layers, adjustments, etc… will be retained. However files exported as .psd may not open and look exactly as they did in Affinity if they contain adjustments, filters, effects, etc… that can not be stored in the .psd format or otherwise found in the other app that is opening them.
And for files containing vector objects, exporting as .svg, .eps and .pdf can be used if you wish to work with their contents in another app.

Go to "Edit > Fill", it´s Shift + f5 by default
 
yes i mean you can set it up in custom shortcuts 

Hey MBd..... can you confirm that? I'm knockin' my head against the wall. AP and AD.
 
(do you mean setting something up in custom shortcuts?)

Jimmy I think cmd + delete is "fill with foreground colour" or something like that
 
You can set this up to be cmd + backspace in AP as well

Or..... do you mean delete to white (or any color)? 
Like it can be done in Photoshop..... cmd+del = delete to foreground color, option+del = delete to background color.
 
We don't have that functionality in AP. I sorely miss it.

Yes. Use the Move tool for that.

I think you want to delete it because it is currently white, right?
 
Go to filter > erase white paper (the easiest way)
 
or use blend ranges to eliminate the white https://vimeo.com/130959696 (more advanced way, gives you more control) 
(just drag down the right side of the curve instead of the left side)
 
further explanations are for example here https://forum.affinity.serif.com/index.php?/topic/17364-blend-if-blend-ranges/?p=79182

You could add a white fill layer below the photo layer, or select the transparent area & use the flood fill (bucket) tool to fill it with white.

you can select the part you want to use to get rid of the people, copy it (ctrl-c) and paste it (ctrl-v). it will be pasted as a new layer.
then select the new layer and the move tool to get it into place and resize as needed. you can apply a mask to it to adjust its borders, fade them, etc.
lower its opacity to place it precisely over the background.

You can use one of the rectangular marquee tool.
Make your selection, then do a cmd J so you have the selection on it's own layer.
Use the move tool to move the selection tho where you want it. 
You may want to tweak it a little. Use the refine selection to feather it. Add a levels (or brightness and contrast) adjustment layer to adjust the contrast..
If you use an adjustment layer, you have to nest it to the selection layer, otherwise the whole picture will be adjusted.