Explorer/Adventurer/Veteran/Champion/Hero are properties of the base item; increasingly referred to as the upgrade track. Each upgrade track has an ilvl cap it will not go beyond and to go higher you need to get a better base item.
However… when you get to the point where you are investing crests, don’t worry about spending them wrongly. There is a discount system such that you never spend the same crest on the same item slot twice.
E.g. you loot 4/8 veteran boots and spend 4 drake crests to get them to 8/8. Later you loot 1/8 champion boots; lower ilvl than your veteran boots, but with a higher cap. The only cost to get them to match your veteran boots is flightstones - no crests. Because you already spent the crests to get some boots of that ilevel. (And flightstones rain on you. Never worry about running out of flightstones.)
Yes there are 5 ranks of upgrade track and yes they overlap. It looks awful on paper; but it’s not actually that bad. Because of the discount mechanism above, the TLDR is that you should never be afraid to use currencies to upgrade an item that is or can be better than what you’re wearing. Your investment is respected, and when something with a higher cap comes along, you can just swap to it and continue along the upgrade track from where you left off.
All you really need to focus on is that tougher challenges reward items that are on higher upgrade tracks. There is a sweet spot at M+11 for relatively little investment and high reward, but you may not be there for a while if you’re worrying about Explorer tier. Don’t worry too much about it yet, just be assured that it’s ok to upgrade things
The system does not work like that which is a shame.You need to find a piece of adventurer, veteran or champion now.There is the 5 heroic dungeons quest that gives a good piece and also the 5 timewalking dungeons quest the same.And the research under fire tier 4 drops one too(do it with a group,will be faster and can reach tier 4 in it).I ignore all explorer gear cause I know its useless in the long run and I have like 30 whelpling crests which probably I ll vendor.
That is as far as the item can be upgraded. However you do have the chance to get the same item in a champion piece and the can further upgrade it to 437 but at that point you will start needing Wyrm Crests.