It is not unusual to find lots of different advice about pruning and training grapevines.  Grapes are remarkably "flexible" in how they can be grown, allowing for quite a bit of adaptation to different situations.  I'll start with some basics.  

If your young vines had a pretty good growing season in 2020, nearly all of the dormant buds on the canes have the potential to produce a shoot with fruit clusters on it in 2021.  The young root systems cannot support the physiological needs of the amount of foliage and fruit that will occur if you do nothing.  That is the principle reason for pruning now and removing some shoots once the growing season begins.  I will come back to this later.

You get to decide where you wish the vines to bear their fruit, based on what sort of a structure you wish to grow the vines on.  In typical vineyards, there is a "bearing wire" that the vine is trained to, but the same purpose can be served by any sturdy structure.  You get to pick the height of the fruit-bearing area to suit your needs, then you grow out the vines to meet the structure.

The shoots that grew last year, which are now canes, can be directed straight upward toward the intended bearing area; if the canes are long enough, they can be gently bent and run laterally along the bearing area (what to do if they are too short comes later).  This sets up the basic vine structure.  All the rest of the pruning and training of the vine for years to come will be done to maintain this basic structure.

The portion of the canes from the base of the vine up to the bearing area are now your "trunks", and you remove almost all of the shoots that grow out from this area, keeping just 2-3 of the lowest ones- these are "spare parts", shoots you will let grow each year, directed straight upward, that can be used to replace injured trunks.  Remove any fruit that develops on these shoots.  If they are not needed, they can always be cut out at pruning time the following year.

The buds on the canes above the trunk, which are in your target fruit bearing area, are the ones that you keep, allowing them to grow out and bear fruit.  

Now we roll ahead to fall of 2021.  Each of the shoots that grew out from the buds on the old canes have now matured into canes themselves.  Each one will have many buds on them, all capable of repeating the process.  If no pruning is done, there will be hundreds of buds on each vine, way too much for the vine to support in the next growing season.  After the vines are fully dormant, but before they break dormancy in 2022, the need to be pruned, to bring them back into a condition of only 20-30 buds in the bearing area.  There are two ways to do this, cane pruning and/or spur pruning.

Cane pruning is really just a "renewal"- you pick a couple of canes that originated from near the top of the trunk to save and put into position in the bearing area; the rest of the old cane new canes are cut off.  The vine now looks like it did at the start of 2021, only the trunk is now more substantial.  If one or more of the trunks is dead or injured, there should be a couple of the "spare parts" canes growing up from the base of the vine to replace what's missing.

Spur pruning is simpler and quicker, but has issues.  Instead of renewing the vine structure by using a couple of good canes originating from the top of the trunk, the canes placed in the bearing area in 2021 are allowed to stay in place.  These are now a year older and called "arms" or "cordons".  They bear all of the one year old canes that grew in 2021.  To spur prune, these one year old canes are cut way back, leaving only 1-2 buds each (a "spur" of 1-2 buds).  This reduces the number of buds and keeps the buds in the bearing area, just like cane pruning, but there is now perennial wood in the bearing area.  

In either system, the trunks and the base of the vines are handled the same way- cleaning the shoots off of most of the trunk and trying to leave 2-3 "spare parts" shoots coming up from the base each year (always removing fruit that develops on these).  You can play the spur pruning game for 3-4 years in a row before the vine becomes quite a mess of old wood.  Most commercial growers alternate between spur pruning and cane pruning, to keep things tidy and healthy.

Now, back to what happens if the vines are currently too short this spring.  Do your best to support the canes heading straight up, tie them to a stake if needed.  After bud break, remove most of the shoots, keeping 5-6 near the top and 2-3 near the base for the spare parts.  Direct the growth of all of these upward, so at least 2-3 of them reach the future bearing area for the 2022 season.  It is OK to keep the fruit on some of the higher shoots, but in order to make sure the vine puts its energy into root and shoot growth, it is best to keep the fruit load small until the next growing season.  

Well, that's a lot to absorb.  Grape vines, once established, are incredibly tough and long-lived plants.  You can manipulate them into whatever "design" you wish, ending up with trunks going around corners, up decorative supports, across arbors, whatever.  The most common error is letting too many buds remain after pruning.  The vines will try to grow out every one, resulting in reduced shoot growth, too much fruit and very poor ripening.