Darkglowing

damn, so many errors, i really appreciate for your help and research you do, in reality im lost i dont even know what im watching right now here, but im eager to learn more about since it will help me in future to recognize such kind of things, so the problem lies with the holes how they were made in the first place. Would it help if the holes and the whole model were merged/grouped as a solid object?; i even saw a video where someone is explaing something about using archicad for 3d printing but i don't know what im searching in the video and if it is explained in it. Maybe something is wrong with de-scaling the object so that the details dissapear? Links to the videos:

So what are the options/solution here to be able to print this model? and what does it mean watertight?

Hello! So a friend asked me if i could print him a model 230mm x 230mm x 150mm he created in archicad 25 program for a project he is on. Because the project is to big in real life and the measurements were in meters he downscale it to fit the printers bed size and exported it as an .stl file. When i open the file, the model loads into cura, but the problems occur instantly, first i get a message about error with the model something with mesh tools that the model isnt watertight but it has holes in it logical. But when i try to slice it everything seems ok but i can not print it with roof upside because i would need to make supports everywhere ( waste of material and costly), so i decided to flip it roof first (on the printer bed), and there occurs the next problem, when i flip the model with the roof on the heat bed and slice it, the program doesen't recognize the holes anymore that were i as design implented. I don't understand same happens if i flip the model on the side wall where the holes are too, the holes simply dissapear but are still showing the position. Can someone check my file and tell me whats wrong with it? maybe even try to fix it so it would print roof first. Or must he change something in archicad before sending me the file Thank you for any help provided! radenci m1_2.stl