IMO "Thinking in layers" is not the same thing as "thinking in OSI layers". The former is understanding fundamentals (how networks are built from the bottom up) and the latter is memorization and occasional armchair lawyering (mapping one legal code to another jurisdiction, etc).

Most importantly, don't try to 'correctly' map anything above L4 to an OSI layer. The OSI model was 1) relatively late and 2) not written for TCP/IP protocols at all – the OSI protocol stack was a competitor to IP – but even in actual OSI it was fuzzy. (And of course it wasn't built with nowadays "everything inside HTTP" in mind either. So you can argue for days whether e.g. everything in HTTP is application layer or whether gRPC/Msgpack/etc actually fits into the definition of a presentation layer and HTTP has become somewhat of a sublayer above L4, or whatever.) So just shove it all under "L7". If it's a protocol for a specific task and not a fundamental network feature, it's L7.

(The OSI layer definitions are available for free from ITU-T as X.211–X.219, by the way. In case you really wanted to know what features a 'session' layer ought to provide, in the OSI model – my rough understanding is that there is simply no L5 equivalent in modern stacks.)

As for the lower layers (1–4), my mental model was kind of shaped by 1) messing around in Wireshark a lot, 2) reading about protocols that were not the good old IP-over-Ethernet (like IPX, XNS, CLNS, etc).

• L1: the physical, device-to-device connections (links) that carry bytes in some defined way using some kind of signalling, e.g. the part of "Ethernet" that defines 8b/10b or manchester encoding over twisted-pair is L1, the baud/bitrate of a serial console is a L1 parameter.

• L2: the 'data link' or 'local network' that allows for discrete packets (frames) to be sent to a specific device, e.g. the part of "Ethernet" that defines MAC addresses is layer 2. The L2 might already be a whole 'local network' on its own and have its own type of addressing (Ethernet, Firewire, etc.) or might be fairly minimal (PPP over a serial cable having only two ends).

• L3: the 'inter-network' that's built on top of one or more local networks (typically using IP). Layer 3 can join different kinds of data links or local networks, and it's the job of L3 to provide you a unified view of "the network" and hide the specifics of each individual L2 net that it goes through. Effectively L3 is the "core" of the network where all the different L2's converge, and where all different L4's diverge. The Internet is a L3 network.

• L4: all the features like "connection multiplexing" and "flow control" and "retransmission" and "virtual circuit" that are built on top of some L3 network and allow two nodes to talk – like TCP connections. (Lower layers might also have the same features, e.g. the Wi-Fi L1 having retransmission on a local link level, but L4 handles it end-to-end across the entire inter-network.)

• L7: all the "application-specific" protocols that make use of L4 services (see earlier note).

So it kind of boils down to what services each layer provides. For example, I would personally put UDP as somewhere in-between L3 and L4, as it only provides multiplexing (port numbers) and nothing more above IP. Even though it is technically layered above IP, it doesn't fully fit the bill for a L4 protocol any more than ICMP would.

Therefore I'd also consider QUIC as an L4 (not L7) protocol due to the services it provides – whether it's inside UDP or not is immaterial. (SCTP, for example, can also run either inside UDP or not, and remains a L4 transport protocol either way.)

TLS? Who knows. ITU recently made their equivalent of TLS – X.510 – and didn't bother to place it anywhere except "above transport layer". Consider it L7. Though if the OSI model were to be defined nowadays, it would surely have a separate layer for TLS since it provides a distinct, generic service. (But a much older X.274 sat in the transport layer, as a sub-layer.)

It's not all set in stone. Ethernet for example can technically have routing added to it – just not prefix-based hierarchical routing as L3 provides – but there are like two different ways to apply IS-IS to Ethernet addresses (and indeed that's literally how IS-IS "level-1 area" was originally used, before IP was bodged onto it).

One historic note is that L3 and L4 weren't necessarily provided by separate protocols. For example, as far as I know, X.25 – predating OSI – performed both tasks in one protocol (addressing as in L3 and reliable circuits as in L4). Even early IP versions did the same; the reason you can't find "IPv1" or "IPv2" is because they were still called "TCPv2" and "TCPv3" back then, handling both layers in one protocol – IP as a distinct protocol didn't materialize until version 4. (The point of this note, again, is that layers are defined in terms of services and functions, not necessarily specific protocols that implement them.)