'J' key is unresponsive

This is tricky! If you have an Apple Store near you I would let them fix this. It often just some junk under the keycap which is stopping the action to work correctly. These KeyCaps can be broken quite easily and as they have them in the back room it’s an easy fix for them. They may charge you a small fee so keep that in mind.

If that’s not possible I would get of can’d air and being very careful a very short blast along the edge should dislodge the debris. You do want to do this with the keyboard upside down! So if falls out not deeper in.

If that fails I would order the parts to have on hand and also referencing them you can see where you need to be careful a guitar pick just under the edge nothing more is what do.

For the replacement KeyCaps we have a bit of an issue as the 2019 16” used a different keycap than the 13 & 15” models. This is the correct one:

Type R Keycaps

Here’s a vid which might help How to replace all keycaps on a MacBook Pro A2141, A2251, A2289 and A2338 Intel & M1 (2019 - 2021)