What is the safest way to install software that it is not on the main repository?

I wanted to know which is the safest way to install applications when this are not on the main repository, I mean safe in the sense of both not unintentionally installing malware and not damaging or creating conflicts within my OS.

I have read some people saying that SNAP could damage your system, others that .deb do, so I'm a bit confused since there are multiple opinions on the subject. Still, I guess that the best way is compiling from source, but if I can't do that, what's next?

I am using Debian 10.