I see two broad options, one of which is a bit of a frame-change.
First, the frame change: most undead - especially low-level undead like zombies and skeletons - were mindless in previous editions of D&D and aren't terribly bright in 5E. It shouldn't be too hard for some passably persuasive denizens to convince the undead hordes to follow their commands without magic. Further, zombies and skeletons are frequently portrayed as basically milling about doing nothing in particular if there's nothing better to do; it shouldn't be too hard for the denizens to have corralled the undead into holding areas for later use (it's not like they need food or exercise, after all).
Option 1b, I guess, would be that there's no particular reason for the undead creatures to stop doing whatever it was that they were doing when they're no longer controlled, especially if what they were doing is a mindless, repetitive task ("keep turning this wheel" or "walk along this round pathway (ie., patrol)").
If that's not quite sufficient, the denizens could be keeping a handful of key undead creatures under their control, and trusting that the rest of them will just kinda follow along.
PCs could, in principle, do the same thing: control or convince (or just lure) the undead to where you need them to be, then let them hang out there 'til they're needed. If necessary, keep a small number under control for specific purposes.
The second option would be to adjust (slightly) the Hallow spell, and "Un-Hallow" the area (which may indicate that a more powerful creature is behind this, and the adventure is prelude). As written, Hallow appears designed for Good PCs to use; I don't see why its mirror shouldn't exist, which would allow for necromancers to exert some control over more undead than normal (possibly only just "some" control, though: perhaps it's just enough that the undead see the other denizens as "friendly" as opposed to "lunch").
Again, in principle a PC could do this, if they could find a deity willing to allow it.
If your intention is to control Large and larger undead: that's pretty easy.
If your intention is to create Large and larger undead: you will either have to wait for WotC to produce the appropriate material, or... get creative with your DM's world.
A wizard of the school of necromancy is given little restriction on their use of their Command Undead feature other than the undead's Charisma and Intelligence (Player's Handbook, pg. 119):
Starting at 14th level, you can use magic to bring undead under your
control, even those created by other wizards. As an action, you can choose one
undead that you can see within 60 feet of you. That creature must make
a Charisma saving throw against your wizard spell save DC.
...
If the target has an Intelligence of 8 or higher, it has advantage on
the saving throw. If it fails the saving throw and has an
Intelligence of 12 or higher, it can repeat the saving throw at the end of every hour until it succeeds and breaks free.
Therefore, any undead you can see can be yours, as long as it is dumb and ugly enough and you are powerful enough.
Unfortunately, there are no spells or abilities published yet that allow creation of giant undead. However, if they exist in the world you inhabit, they must come from somewhere, right?
Therefore, your first task would be to find out what makes giant undead creatures in the world. Find out what is creating them and where:
- Is it an NPC necromancer who stumbled across an ancient artifact that can amplify the effects of necrotic spells (as in my campaign)?
- Is there a haunted graveyard where the restless dead can't help but stretch their legs? (common trope)
- Or is there a particularly unholy bog somewhere nearby that just imbues corpses in it with undeath?
Once you find that answer, simply harness that energy, adapt it to meet your needs, and you will be churning out giant undead slaves in no time.
Best Answer
There are four spells which make undead:
Animate Dead. 3rd level spell which is added to your spellbook at 6th level and for which you get an additional undead. Makes zombies or skeletons.
Danse Macabre. 5th level spell from Xanathar's Guide to Everything (p. 153) which creates 6 undead for an hour. Makes zombies or skeletons, with a bonus to attack and damage.
Create Undead. 6th level spell which creates 3 ghouls which last indefinatly. Can be cast at higher levels to make 4 ghouls (7th), 5 ghouls or 2 ghasts or wights (8th) or 6 ghouls, 3 ghasts or wights, or 2 mummies (9th). Must be cast at night for added cliche. Lightning strike and full moon optional.
Finger of Death. 7th level spell which can create a zombie of its target if it kills it. The zombie has no time limitations or upkeep costs.
These are buffed by Undead Thralls
The 6th level feature which grants you the animate dead spell, Undead Thralls, also buffs these undead:
Which is a buff to their hit points and damage (this stacks with the bonus innate to danse macabre).
There are otherwise no other features (to my knowledge) which explicitly improve the stats of your undead. (Obtaining minions in 5e is already quite powerful as it weighs heavily on the action economy.) You are then left with the myriad of spells and features which can buff any creature. I'm not going to list them here, but you may find D&D Beyond's Spell Search a useful starting point to find buff spells.