This post shows how to create a simple SharePoint 2016 Page layout without using:

SharePoint designer
Design manager

Before doing anything, you need to create the base of the layout.

I have found the following placeholders are minimal and commonly used in any template.

PlaceHolderAdditionalPageHead
PlaceHolderPageTitle
PlaceHolderMain

Place Holders

PlaceHolderAdditionalPageHead

This place holder is used to define additional CSS and other scripts and inline CSS if you have any.

I will be using this to register a CSS and register a different CSS for the SharePoint Edit Mode.

<asp:Content ContentPlaceHolderId="PlaceHolderAdditionalPageHead" runat="server">
<!—Your code goes here-->

</asp:Content>

PlaceHolderPageTitle

This placeholder will be mainly for the page title.

<asp:Content ContentPlaceHolderId="PlaceHolderPageTitle" runat="server">
     <!—Your code goes here-->
</asp:Content>

PlaceHolderMain

This is the main section for your temple. Here, you can define web part zones or any other HTML code segments such as Bootstrap or static HTML as you want.

 <asp:Content ContentPlaceHolderId="PlaceHolderMain" runat="server">
<!—Your code goes here-->
</asp:Content>

Base Template

Adding standard SharePoint controls, you can make your base template as follows:

<%@ Page Language="C#" Inherits="Microsoft.SharePoint.Publishing.PublishingLayoutPage,
Microsoft.SharePoint.Publishing,Version=15.0.0.0,Culture=neutral,PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="SharePointWebControls" Namespace="Microsoft.SharePoint.WebControls" 
Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="WebPartPages" Namespace="Microsoft.SharePoint.WebPartPages" 
Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="PublishingWebControls" 
Namespace="Microsoft.SharePoint.Publishing.WebControls" 
Assembly="Microsoft.SharePoint.Publishing, Version=15.0.0.0, Culture=neutral, 
PublicKeyToken=71e9bce111e9429c" %>

<asp:Content ContentPlaceHolderId="PlaceHolderAdditionalPageHead" runat="server">
         <!-- Styles for edit mode only-->
</asp:Content>

<asp:Content ContentPlaceHolderId="PlaceHolderPageTitle" runat="server">
     <!-- Styles for edit mode only--> 
</asp:Content>

<asp:Content ContentPlaceHolderId="PlaceHolderMain" runat="server">
     <!-- Styles for edit mode only-->
</asp:Content>

Extend Your Template

PlaceHolderAdditionalPageHead

First, I’m going to add general CSS registration to this section:

<SharePointWebControls:CssRegistration 
name="<% $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/pagelayouts15.css %>" 
runat="server"/>

Then, I'm going to define a CSS for EDIT mode, this will be really needed when you are hiding some tags when not in the Edit Mode.

<PublishingWebControls:EditModePanel runat="server">
     <!-- Styles for edit mode only-->
     <SharePointWebControls:CssRegistration name="<% 
     $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/editmode15.css %>"
         After="<% $SPUrl:~sitecollection/
         Style Library/~language/Themable/Core Styles/pagelayouts15.css %>" runat="server"/>
</PublishingWebControls:EditModePanel>

Then, I will add other scripts or CSS formatting if necessary.

    <style type="text/css">
         <!-- Styles -->
     </style>
     <script type="text/javascript">
         // your code
      
     </script>

Finally, you will have your AdditionalPageHead as below:

<asp:Content ContentPlaceHolderId="PlaceHolderAdditionalPageHead" 
runat="server">
  <SharePointWebControls:CssRegistration name="<% 
  $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/pagelayouts15.css %>" 
  runat="server"/>
     <PublishingWebControls:EditModePanel runat="server">
         <!-- Styles for edit mode only-->
         <SharePointWebControls:CssRegistration name="<% 
         $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/editmode15.css %>"
             After="<% $SPUrl:~sitecollection/Style Library/~language/
             Themable/Core Styles/pagelayouts15.css %>" runat="server"/>
     </PublishingWebControls:EditModePanel>

    <style type="text/css">
         <!-- Styles -->
     </style>
     <script type="text/javascript">
         // your code
      
     </script>
</asp:Content>

PlaceHolderPageTitle

I’m just adding the SharePoint Page Title here.

<asp:Content ContentPlaceHolderId="PlaceHolderPageTitle" runat="server">
     <SharePointWebControls:FieldValue FieldName="Title" runat="server"/>   
</asp:Content>

PlaceHolderMain

Here goes with one web part zone for simplicity.

<asp:Content ContentPlaceHolderId="PlaceHolderMain" runat="server">
       <WebPartPages:WebPartZone runat="server" Title="Top Left" 
       ID="TopLeftZone"><ZoneTemplate></ZoneTemplate></WebPartPages:WebPartZone>
</asp:Content>

Final Template

<%@ Page Language="C#" Inherits="Microsoft.SharePoint.Publishing.PublishingLayoutPage,
Microsoft.SharePoint.Publishing,Version=15.0.0.0,Culture=neutral,PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="SharePointWebControls" Namespace="Microsoft.SharePoint.WebControls" 
Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="WebPartPages" Namespace="Microsoft.SharePoint.WebPartPages" 
Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="PublishingWebControls" 
Namespace="Microsoft.SharePoint.Publishing.WebControls" 
Assembly="Microsoft.SharePoint.Publishing, Version=15.0.0.0, 
Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>

<asp:Content ContentPlaceHolderId="PlaceHolderAdditionalPageHead" runat="server">
     <SharePointWebControls:CssRegistration name="<% 
     $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/pagelayouts15.css %>" 
     runat="server"/>
     <PublishingWebControls:EditModePanel runat="server">
         <!-- Styles for edit mode only-->
         <SharePointWebControls:CssRegistration name="<% 
         $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/editmode15.css %>"
             After="<% $SPUrl:~sitecollection/Style Library/~language/Themable/Core Styles/
             pagelayouts15.css %>" runat="server"/>
     </PublishingWebControls:EditModePanel>

    <style type="text/css">
         <!-- Styles -->
     </style>
     <script type="text/javascript">
         // your code
      
     </script>
</asp:Content>

<asp:Content ContentPlaceHolderId="PlaceHolderPageTitle" runat="server">
     <SharePointWebControls:FieldValue FieldName="Title" runat="server"/>   
</asp:Content>

<asp:Content ContentPlaceHolderId="PlaceHolderMain" runat="server">
   <WebPartPages:WebPartZone runat="server" Title="Top Left" 
   ID="TopLeftZone"><ZoneTemplate></ZoneTemplate></WebPartPages:WebPartZone>
</asp:Content>

Create a Custom Layout for SharePoint 2016 and Upload to the Page Layout Library– Part 1