the question is good and not that simple to answer.
There are many antennas described on the web and it is easy to replicate some of them. But in most cases there are no detailed explanations and it is difficult to modify them without such details.
Okay, say you want to build a multiband End fed EFHW. You need:
a) a feeder
b) a run of wire for each band, and
c) a LC trap if you want automatic band change.
Let’s go little by little.
You can build a simple feeder that will run properly for all HF bands. You don’t need to change it when moving from one band into another.
If you change from SSB to CW, again, you can keep the same feeder.
The transformer described here is a good one:
If you intend to run QRP, you can build a smaller one changing the toroid, for example using a FT-114-43.
If your feeder is properly built you won’t need to use a Tuner at all in the rig.
Once you have built the feeder, you can cut the first run of wire, for the highest band of your choice. If it is 18 MHz then cut about 1/2 wavelenght for 18.050 MHz. There are onliine calculators to give you the number. Consider the antenna wire is covered with a PVC cover that reduces the actual length a bit (velocity factor effect).
So one you have the wire, install it on the field the way you will use it in the mountain (inverted vee? vertical?) and use your rig to moniitor the SWR. Then trim your wire untill you get a proper SWR. If you do it properly you will be able to use this wire both for SSB and CW, because the bandwidth is enough for both.
On 14 MHz the variation is longer(14-060 for CW - 14.285 for SSB) but if you tune your wire to be resonant in the middle, say 14.150, you coulld achieve acceptable SWR for both modes.
Then you shoud add a Trap now, to do the band change, right?
C) LC TRAP
Many people use just links. The band change is then performed manually, and that requres moving the wire up and down to have the link at hand.
But you prefer the automatic band change and therefore you want traps. Okay.
I have described the process of calculating traps here, but I’m afraid it is in spanish!
I understood you bought some commercial made traps. Then follow their data-table to adjust them for each band.
If you move between SSB and CW for a single band, you need not to change anything in the traps. They are normally tuned in the lower portion of the band (i.e. for 18 MHz they should be resonant at 17.900 or so, to work best).
When the trap is ready, add it to your first wire and now add a second wire as an extension for the new band. Total length (first + second) should be roughly 1/2 lambda for the new band. Again, use you rig to monitor SWR and trim the extension wire.
This process is followed again and again for each band.
MODE CHANGE (CW - SSB)
If you change between SSB and CW the important point is the wire, not the trap. So if you want to fine tune the SWR when moving within the band, all you could do is to collapse the wire a little bit on the feed point. if you make a little roll in the feed point it will shorten the total length of wire and therefore the resonance will be better for SSB. To move to CW you would do opposite, to extend the wire in the feed point.
In my experience, I normally do not need to do anything at all. When I QSY between SSB and CW in a band I see a little variation of SWR, but you can still work fine with your rigs until a reading of 1,5 or 2. Even 3 when required. We won’t need to get too obsessed to get a perfect 1:1 value all the time.
In the past I sometimes carried a spare length of wire (about 30 cm) that I could connect and disconnect easily in the feed point to enlarge the antenna length,
But I changed now my antenna by adding more length to make it resonant in CW and now it works right both CW or SSB, with little the SWR variation described.
An additional observation, when activating in one or another mountain, I also get different readings of SWR. The terrain and variations of the installation modify slightly the SWR. Again, don’t get too obsessed with that. Keeping the feeder well above the soil is a good advise (1m height ±).
Hope it helps a bit with your questions.
Good luck and 73 de Ignacio