How to close gaps on sliding door rail and install weatherstrips under the doors?

I have this sliding door that was getting hard to opening and close so I called someone to come and service it (replace rollers, clean rail... that sort of stuff).

Now it opens and closes just fine but I think the guys left me with even bigger problems.

I noticed these two big openings on each side of the rail (see pictures). Is this normal or expected? How can I close them? Do I need to replace the rail? These holes are big enough for me to get all sort of pests inside the house.

Other than that, door is now leaking a considerable amount of air. I get my hand 5 inches from the bottom of the door and I can already feel the AC coming out.

As far as the gap on the edges goes I'm guessing it was covered on dirty so I didn't see it before. As for the leaking air, I saw a bunch of these stuff on the floor that looks like weatherstrips, looks like they just removed then - go figure. If this is the case, how can I install weatherstrips under the door and what kind strips should I buy?

Left lower corner:

Right lower corner:

Source: https://diy.stackexchange.com/questions/148773/how-to-close-gaps-on-sliding-door-rail-and-install-weatherstrips-under-the-doors