Summoned some Villagers with the same trades they normally offer. Turned into Zombie Villager, then cured, but their trades stayed the same. Tested with a Master level Mason with 10 trades and an Apprentice Librarian with 4 trades. Tested in Multiplayer server and Single player

Summoned with Command Blocks in Creative mode. Turned into Zombie Villagers and cured in Survival mode

Update after testing for 1.16rc1

• Villager seems to be getting experience from trading with the player very slowly. Villager level doesn't seem to match xp needed to advance the level. After trading for 120 xp worth, Apprentice level bar moved forward about 30% of the total, when it should only require 60 xp to level up to Journeyman
• Command in Command Block used to summon villager:

  summon villager ~ ~1 ~ {VillagerData:{profession:librarian,level:2,type:plains},Offers:{Recipes:[{buy:{id:paper,Count:24},sell:{id:emerald,Count:1},maxUses:16},{buy:{id:emerald,Count:5},buyB:{id:book,Count:1},sell:{id:enchanted_book,Count:1,tag:{StoredEnchantments:[{id:silk_touch,lvl:1}]}},maxUses:12},{buy:{id:emerald,Count:1},sell:{id:lantern,Count:1},maxUses:12},{buy:{id:emerald,Count:16},buyB:{id:book,Count:1},sell:{id:enchanted_book,Count:1,tag:{StoredEnchantments:[{id:feather_falling,lvl:4}]}},maxUses:12}]}}