Your question is not really clear, but when you want to look at the standard onet.xmlor webtemp.xml files you just have to check the 15-hive.
The onet.xml for the Standard Team site is located under:
C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPlATE\SiteTemplates\sts\xml
Two important notes about this: Under SiteTemplates you actually find all of the Standard Site templates. sts in this case represents the team site. You might also want to check out the features folder to see all of the supplied features.
Based on the STS-template you can create your own template and take ideas from the standard.
To see all available site templates and their names try the following in PowerShell:
get-spweb http://<yoursharepoint> | %{$_.GetAvailableWebTemplates(1033)} | select title, name
I get the following output:
Title Name
----- ----
Global template GLOBAL#0
Team Site STS#0
Blank Site STS#1
Document Workspace STS#2
Basic Meeting Workspace MPS#0
Blank Meeting Workspace MPS#1
Decision Meeting Workspace MPS#2
Social Meeting Workspace MPS#3
Multipage Meeting Workspace MPS#4
Central Admin Site CENTRALADMIN#0
Wiki Site WIKI#0
Blog BLOG#0
Group Work Site SGS#0
Tenant Admin Site TENANTADMIN#0
App Template APP#0
App Catalog Site APPCATALOG#0
Access Services Site ACCSRV#0
Access Services Site Internal ACCSVC#0
Access Services Site ACCSVC#1
Document Center BDR#0
Developer Site DEV#0
Academic Library DOCMARKETPLACESITE#0
eDiscovery Center EDISC#0
eDiscovery Case EDISC#1
(obsolete) Records Center OFFILE#0
Records Center OFFILE#1
Shared Services Administration Site OSRV#0
PerformancePoint PPSMASite#0
Business Intelligence Center BICenterSite#0
SharePoint Portal Server Site SPS#0
SharePoint Portal Server Personal Space SPSPERS#0
Storage And Social SharePoint Portal Server Personal Space SPSPERS#2
Storage Only SharePoint Portal Server Personal Space SPSPERS#3
Social Only SharePoint Portal Server Personal Space SPSPERS#4
Empty SharePoint Portal Server Personal Space SPSPERS#5
Personalization Site SPSMSITE#0
Contents area Template SPSTOC#0
Topic area template SPSTOPIC#0
News Site SPSNEWS#0
Publishing Site CMSPUBLISHING#0
Publishing Site BLANKINTERNET#0
Press Releases Site BLANKINTERNET#1
Publishing Site with Workflow BLANKINTERNET#2
News Site SPSNHOME#0
Site Directory SPSSITES#0
Community area template SPSCOMMU#0
Report Center SPSREPORTCENTER#0
Collaboration Portal SPSPORTAL#0
Enterprise Search Center SRCHCEN#0
Profiles PROFILES#0
Publishing Portal BLANKINTERNETCONTAINER#0
My Site Host SPSMSITEHOST#0
Enterprise Wiki ENTERWIKI#0
Project Site PROJECTSITE#0
Product Catalog PRODUCTCATALOG#0
Community Site COMMUNITY#0
Community Portal COMMUNITYPORTAL#0
Basic Search Center SRCHCENTERLITE#0
Basic Search Center SRCHCENTERLITE#1
Visio Process Repository visprus#0
As you can see, "Document Center" is BDR#0, hence the path is: C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\SiteTemplates\BDR
