Use FLUIDTEMPLATE cObject and bind page's backend layout to template file

templates.ts

# Initiate template (used by page)
lib.templates.base = FLUIDTEMPLATE
lib.templates.base {
	partialRootPath = EXT:typo3sz_assets/Resources/Default/Private/Templates/Partials/
	layoutRootPath = EXT:typo3sz_assets/Resources/Default/Private/Templates/Layouts/

	variables {
		logo =< lib.contents.logo

		navMain =< lib.navigations.main
		navSub =< lib.navigations.sub
		navService =< lib.navigations.service
		navBreadcrumb =< lib.navigations.breadcrumb
		navLanguage =< lib.navigations.language

		columnLeft =< lib.contents.columns.left
		columnMain =< lib.contents.columns.main
		columnRight =< lib.contents.columns.right

		contentBefore =< lib.contents.before
		contentAfter =< lib.contents.after

		footerCopyright =< lib.contents.footerCopyright
	}
}

# Choose template file (based on backend_layout, respecting heredity)
lib.templates.base.file.stdWrap.cObject = CASE
lib.templates.base.file.stdWrap.cObject {
	key.field = backend_layout
	key.ifEmpty.data = levelfield : -1 , backend_layout_next_level, slide

	default = TEXT
	default.value = EXT:typo3sz_assets/Resources/Default/Private/Templates/Main.html

	typo3sz_base__main-right = TEXT
	typo3sz_base__main-right.value = EXT:typo3sz_assets/Resources/Default/Private/Templates/MainRight.html

	typo3sz_base__main-right-disabled = TEXT
	typo3sz_base__main-right-disabled.value = EXT:typo3sz_assets/Resources/Default/Private/Templates/Main.html

	typo3sz_base__left-main-right = TEXT
	typo3sz_base__left-main-right.value = EXT:typo3sz_assets/Resources/Default/Private/Templates/MainLeftRight.html
}