You assume incorrectly. The contractor is still responsible for sourcing trades, QC supervision, and coordinating and scheduling all of the work around the cabinet, counter, backsplash, and appliance install. Those are fingers of the fist, and the GC is the palm in the middle. He still gets his 20% even on owner supplied materials. Or else you become the GC and do all of that for the entire build. Which isn’t happening with the bank.

Employee discounts do NOT apply to anything special order. Like cabinets. Only stock items. Which is NOT Schuler. If someone in the cabinet department is rigging some special discount, both employees can lose their jobs. Plus, 85% of the “designers” they have won’t have a clue how to use a cabinet line of that level to its fullest potential. You really should check out Medallion from a dealer. It’s the same thing, only better. It will likely be cheaper, without any potential job losing special discounts, and comes with someone with more experience working with it.

"I am assuming that the allowance for cabinets can be applied towards my purchase of cabinets from Lowe's."

Never assume, trust but verify. How this will be handled needs to be writtne very clearly in the contract and discussed with the builder upfront (in other wrds long before now). If this is a developer it may end up costing you MORE to use use your cabinets. Will he allow you to supply them? You may end up buying his and yours (meaning the allowance just goes away with nothing to you plus you pay for his oversite and extra for install).

It depends on the signed contract. Discuss with GC & get any changes in writing so everyone knows their roles & expectations & costs.

Kitchens are most expensive room and complex room in a home with loads of detailed planning required. Better get with your GC ASAP and clarify expectations and precisely how this will work. Allowances are merely “place holders” until details are defined. Rarely work well. Also, nearly all trades are involved. So, read your contract. It is one thing to have an “exception” to say use a carpet place your brother owns and a kitchen. So many ways this can go wrong. Talk to your GC.