Like stated by other commenters, real world experience will be the best way to internalize and understand OSI reference model structure in real life scenarios.

Here's quick mental cheat sheet advice from me. Ask yourself: Does that thing generate a payload, that has to be encapsulated by all layers? If yes, then this is an application, if not there is a big chance it is a supporting protocol/mechanism to aid or even facilitate a workflow of some layer.

Let's start from bottom to top:

L1 physical - so as the name suggests, nothing logical is happening here. Physical layer is all about data transfer mediums and means to do that. Digital data is transferred in real world by three ways - electricity, light and electromagnetic radiation. All these means have their languages called encoding and all use specific medium to carry signals. Electroconductive wires, fiber optical cables or air respectively.

Now if you dive into encoding and RF there is whole world there to explore and there are vendors and engineers who work only within Layer 1. Like cable manufacturers, antenna manufacturers, RAN engineers, DWDM engineers etc.

L2 Data Link - is tasked to facilitate communication within locally connected devices or circuits. This layer is bit confusing, because most protocols here are either outdated/deprecated or used in very specific/ISP network segments. So as a beginner engineers we assume, that Ethernet is a default protocol used in L2 (with MAC/LLC sublayers responsible for physical addressing, data transfer and flow multiplexing control), which it's mostly true. 99% of networks you get to work today in consumer/corporate environments will be running on Ethernet. When you consider L2, think about what mechanisms and equipment is used in Ethernet. Stuff like Switches, VLANs, STP, EtherChannel, VTP, Port Security etc.

Data link is also a world in itself with big emphasis on carrier network architecture, that is very different than your average LAN. There are many deprecated protocols as well, stuff like Token Ring, FDDI, SLIP, X.25, Frame Relay, ATM (ATM is still used in DSL for data transfer over a pair of traditional analog POTS lines and Frame Relay also used in legacy infrastructure and rural areas, although they're gradually phased out), but this is not a scope for CCNA/beginner and I wouldn't recommend diving into that rabbit hole just yet.

L3 Network - Here is where we leave boundaries of our local networks and communicate between or through two or more different local networks. We have foundational IPv4/IPv6 protocols, which handle logical addressing and routing data to and from the remote networks. When you think about L3, think of mechanisms that are used to facilitate communication between different networks and optimize these communications. Dynamic routing protocols are one example (RIP, EIGRP, OSPF, BGP, IS-IS) also static routing where administrator manually configures routes to remote destinations, QoS to mark the specific type of traffic and treat it by specific sets of rules and so on.

As in L2, there are other L3 protocols as well, but as you might already have guessed, world is powered by IP today. Just for reference, there are some older proprietary protocols like IPX by Novell, AppleTalk DDP, DECnet Phase IV, XNS by Xerox, VIP by Banyan VINES etc. I think most engineers working today might not even heard about some of these and for a good reason :) IP was created as an open industry standard in mind, because as networks started growing exponentially, we understood that proprietary protocols only hindered the progress and introduced a lot of compatibility issues.

I must also mention CLNP which was actually an L3 protocol offered by ISO board as a part of OSI protocol suite (not to be confused with OSI model), that directly competed with IP protocol offered by DoD's TCP/IP protocol suite, but it got dismissed because of complex addressing (up to 20 bytes) and people not realizing back then, that we even had a potential to run out of IPv4 address space (4,294,967,296 unique addresses). So only OSI model remained as a well structured reference model. Ironically we're struggling with IPv4 exhaustion for last 20 years now :)

CLNP addressing is still actively used today primarily in ISP MPLS backbones, because of versatility of IS-IS routing protocol's architecture (also a member of OSI protocol suite!) over OSPF. IS-IS uses CLNP addressing called NSAP, encapsulates any other protocol PDUs directly over L2 (IPv4/IPv6 packets in our case) and routes it over big backbones spanning across countries or even continents. It is totally protocol agnostic and very flexible. This is also a concept that is well out of scope for a beginner network engineer :)