Ventilation Questions

I’m getting a new roof put on in a week and was poking around in the attic to look at a couple things and saw that my kitchen and bathroom fans are both run straight across the ceiling (in the attic) to the soffit vents. They are just the white flexible hoses that are kind of pointed towards the soffit vent (not fastened down or anything). I don’t see how this can be working effectively, although one person I talked to liked this idea because it was two less penetrations thru the roof and hence less chance for leaks.

Should I change these to go thru the roof? If so, what kind of vent cover do you recommend? Also, is it best for each to go straight up and out or can I run one to the other (about 15 feet apart) and then go out through only one roof vent (to cut down on roof penetrations)?

Finally, this might be a dumb question, but the bathroom fan is about a foot away from a plumbing stack; could I tie the two together to go thru the roof only once? Wouldn’t any plumbing gases still rise straight up and out, especially if I angled the bathroom fan vent up towards the plumbing stack? Again, could you recommend a good roof vent for this purpose?

Any and all suggestions are much appreciated! Thanks very much!