I know how the focus points are painted on the mirror but where do the "signs" come from?
How are auto focus points are illuminated red in Canon 1300D?
Where and how are the shutter speed numbers etc shown on the bottom of the viewfinder?
Is there an super small screen under the DSLR mirror where auto focus indicators and f stop number etc indicators are located?
I would love to see images and explanations.
I know how the focus points are painted on the mirror..
Not really, because I've never seen focus points or anything else (intentionally) painted on a reflex mirror. It would be out of focus. Technically, those that do it the way you think you are describing etch them onto the focusing screen, which is located in the roof of the mirror box.
But other cameras now use translucent thin LCD overlays to display information inside the image frame part of the viewfinder. The red dots can be projected onto the apparent location of the focusing screen (a/k/a the view screen) as seen through the viewfinder from beside or above it in the prism housing. The information on the edges around the image frame can be displayed using a variety of methods. Old film cameras actually had mechanical needles in there that displayed metering information. Most of it now is done using LCD or Electroluminescent displays.
Canon started using translucent LCD overlays with the EOS 7D in 2009.
From the Canon EOS 7D review at The-Digital-Picture
It's not clear if that technology has filtered down to the Rebel/xx00D series even today, much less by 2016 when the EOS Rebel T6/1300D was introduced. Based on an image I found of the 1500D's viewfinder, it probably has not. All of the information shown is possible without an LCD overlay. The Rebel Txi/xx0D series does appear to now use transmissive LCD technology in the viewfinder.
Is there an super small screen under the DSLR mirror where auto focus indicators and f stop number etc indicators are located?
No. The display mechanisms are located in the viewfinder housing above the focusing/view screen that is the roof of the mirrorbox. Light from the lens is bounced off the mirror and focused on the viewscreen. What you see in the viewfinder is the image formed by the light focused on the viewscreen, plus whatever display information is projected onto the viewscreen¹ from above, created by a thin, clear LCD placed directly above the viewscreen, or by other types of display placed just above or beside the inverted image on the viewscreen.
¹ Whether the light from the LED projectors is actually projected onto the viewscreen itself, as some cameras do it (see this illustration of the EOS 1N film camera from this article) or projected more directly to the viewfinder exit pupil at the apparent position of the viewscreen as seen through the viewfinder, like other camera's do it (see this illustration of the Nikon FM film camera viewfinder, or the 1D X Mark II illustration below), is immaterial. The effect when observed by looking onto the camera's viewfinder is the same.
This camera cutaway (not your EOS 1300D - it's an Olympus E-400 - but many other DSLRs are similar) shows an information screen (circled in yellow) which emits light that is projected by lenses (circled in red) through the prism and which ultimately appears to be below the bottom of the focusing screen as you see it in your viewfinder:
Some cameras use a hybrid of both methods:
Detail of the Canon EOS 1D X Mark II viewfinder/metering/AF system from that camera's review at The-Digital-Picture.
For a more in-depth look at the various mechanisms different cameras from different eras have used to show information in the viewfinder, please see this two-part article:
Viewfinder Optics Part I
Viewfinder Optics Part II
This is answered in detail in another post: How are the red focus point indicators displayed on a DSLR's focusing screen?
However, all of the DSLR cameras I am familiar with (OVFs) use an overlayed display in a manner similar to the image Michael posted. I updated that image to better explain how that works (some cameras use separate mirrors as shown in the linked post).
In these cases the information is not being projected onto the focus screen.
