Well, again "interval" means "space between", if you want trains to follow each other 100 miles apart you place a signal every 100 miles, if you want train 2 to get a yellow when train 1 pulls ahead 53 1/2 feet, you place the signals 53 1/2 feet apart. It also varies according to switches, got a switch with a signal controlling the switch 33 feet before the next interval would be placed it don't make much sense to stick the interval where it would normally go.
As for the other questions, at some point you'll need to read the manual and the tutorials, unless you're looking for someone to come over and build your route for you.
Content search filter, type "gantry", select the track objects tab and see what you get.
Select the tool tab, select a ruler, stretch it out to 5280 feet. Want a half or quarter mile instead, Windows comes with a built in calculator so you don't even need to do the math. Note here the two outside tracks are signaled for right hand running, the center track is bidirectional with a signal facing both ways on the signal bridge. Main purpose of signal bridges and gantry signals is this type of configuration, two tracks you can place signals on the outside, three or more tracks with not enough space between tracks for signals you move them up overhead.
This junction they're spaced far enough apart to use regular pole signals, note a double head before each facing point switch to show the two possible routes thru the switch. On the trailing point the signal just indicates if the switch is set for you or against you, only need a single head there since there's only one way you can go.
