In order to build a Zygarde at the Lab on Route 16, you must first collect the Zygarde Cells found throughout the entire Alola Region. There are 95 different cells to collect, and the number of which you use at the Lab on Route 16 will dictate what Zygarde Form you will/can build (10% Form, 50% Form, Zygarde Complete). Some of which are found at certain times of the day in-game.
There are also five given Zygarde Cores that can be found scattered throughout The Alola Region, one on every island except Melemele, which as two. These cores are needed in the end if you want to build a Zygarde with the ability of Power Construct. Each core also has a given move associated with it that will be passed on to the Zygarde you build.
All 95 Zygarde Cell Locations:
Melemele Island Zygarde Cells (#1-14)
1. Route 1 (Day)
2. Route 1 (Day)
3. Route 1
4. Route 1
5. Route 2 (Night)
6. Route 3 (Day)
7. Route 3
9. Hau'oli City Pokecenter (Night)
10. Hau'oli Shopping District
11. Iki Town (Night)
12. Ruins of Conflict
13. Kala'e Bay
14. Verdant Cavern
Akala Island Zycarde Cells (#15-32)
15. Route 4
16. Route 5
17. Route 7
18. Route 8
19. Route 8 (Day)
20. Akala Outskirts
21. Diglett's Tunnel
22. Hano Beach (Day)
23. Hano Beach (Day)
24. Heahea City (Night)
25. Heahea City Mansion
26. Konikoni City (Night)
27. Lush Jungle Cave
28. Paniola Ranch (Day)
29. Paniola Ranch (Night)
30. Royal Avenue (Day)
31. Royal Avenue
32. Wela Volcania Park
Ula'ula Island Zygarde Cells (#33-62)
33. Route 10 (Day)
34. Route 11 (Night)
35. Route 12 (Day)
36. Route 12 (Night)
37. Route 13
38. Route 13 (Night)
39. Route 14 (Night)
40. Route 14
41. Route 15
42. Aether's House (Day)
43. Route 16 (Day)
44. Route 17
45. Route 17
46. Blush Mountain
47. Hokulani Observatory (Night)
48. Malie City (Day)
49. Malie City Outer Cape (Day)
50. Malie City (Night)
51. Malie Garden
52. Mount Lanakila Top
53. Mount Lanakila Bottom
54. Po Town Trucks (Day)
55. Po Town Trucks (Night)
56. Haina Desert
57. Haina Desert
58. Secluded Shore (Day)
59. Secluded Shore (Night)
60. Tapu Village
Poni Island Zygarde Cells (#63-90)
63. Ancient Poni Path (Day)
64. Ancient Poni Path (Day)
65. Ancient Poni Path (Night)
66. Ponie Braker Coast (Day)
67. Ponie Braker Coast (Night)
68. Poni Grove
69. Poni Grove
70. Poni Plains (Day)
71. Poni Plains
72. Poni Plains (Night)
73. Poni Meadow
74. Resolution Cave (Day)
75. Resolution Cave
76. Resolution Cave (Night)
77. Poni Coast
78. Poni Coast
79. Ruins of Hope
80. Poni Gauntlet
81. Poni Gauntlet
82. Seafolk Village
83. Seafolk Village
84. Poni Wilds (Day)
85. Poni Wilds
86. Poni Wilds (Night)
87. Vast Poni Canyon
88. Vast Poni Canyon
89. Vast Poni Canyon
90. Vast Poni Canyon
Aether Paradise Zygarde Cells (#90-95)
91. Aether Paradise Hallway
92. Aether Paradise Outside
93. Aether Paradise Outside (Day)
94. Aether Paradise Main Building
95. Aether Paradise Entrance (Night)
All 5 Zygarde Core Locations:
1. Melemele Island - Route 1 (Extreme Speed)
2. Melemele Island - Iki Town (Thousand Arrows)
3. Akala island - Konikoni City (Dragon Dance)
4. Ula'ula Island - Route 17 (Thousand Waves)
5. Poni Island - Ancient Poni Path (Core Enforcer)