If I install macOS Mojave or Catalina on an external HD, will this change my internal HD filesystem?

Cheers to everybody.

I own an iMac Retina 5K 27-inch 2017 (iMac18,3), running macOS 12.12.x Sierra from its internal HFS+ 3 Tb Fusion Drive, but I will likely upgrade to macOS 10.13.x High Sierra in the next days. I am simply testing my “old” software just to see if it works fine under High Sierra, so just doing my homework. But I postponed from day one since I was afraid of changing the filesystem of my Fusion Drive which I want to keep in HFS+. After having checked that High Sierra installer would not change my internal Fusion Drive filesystem at all, I am fine with the upgrade after all.

Now, my strictly related question: should I ever wanted to test macOS 10.14 Mojave instead, but INSTALLING IT ON AN EXTERNAL HARD DRIVE, will its installer possibly change my internal Fusion Drive Filesystem from HFS+ to APFS (which is something that I definitely want to absolutely avoid doing)? I guess it will not, but I really want to be on the safe side before doing this, hence my question here to you guys and the community.

By the way, it might seem strange, but I cannot find any info googling around. It's likely a matter of searching with the correct words (thing that I am clearly incapable of doing, as it seems), but still… I am stuck without any answer. Hoping to receive useful ones from you.

Thank you very much indeed.