I started with a page template in a theme directory, the way I did it in D7...
That's a good start, but you'd need to rename the template file. (The file named
page.tpl.php in Backdrop is the same as the file named
html.tpl.php in Drupal 7)
On the forum i was told to create template in layout directory instead
That's almost right, you'd need to create an entire layout (including an
.info file and maybe CSS) not only the template file. You can copy (or rename) your Drupal 7
page.tpl.php file for the layout template file (
layout--NAME.tpl.php) if you want it to be nearly identical. You'd then need to remove the header, footer, menu, and anything else you'd like to replace with a block in Backdrop.
I have a feeling that things work the other way round compared to last time.
No, it works the same now in 1.11 as id did before.
Does it mean I can remove layout files altogether and just keep coding template in the theme directory?
Yes! I do this quite frequently. You'll first need to find out which layout is most similar to what your Drupal 7 theme did for it's layout. Then you can override that Layout template file in your theme instead of creating an entirely new layout.
For example, if your Drupal 7 site had a left sidebar, you could use the Moscone layout in Backdrop. Then you would copy the
layout--moscone.tpl.php file (from
core/layouts/moscone/) into your theme. Then you could copy what you wanted to keep from your Drupal 7
page.tpl.php file into that
You can visit
admin/structure/layouts/settings to see all the layouts core provides.