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I saw the STP on layer 2. The guy that made the tutorial says at the end :

Even though it seems that Layer 2 access layer solutions are slowly
being replaced by blazing fast Layer 3 solutions.

But when we are routing with IP on layer 3, we are then going layer 2->1 and then 1->2->.. so do we use lower layers protocols ?

Or are they used only for LAN routing ?

Layer 3 (mostly IP) generally relies on the underlying layer-2 network (mostly Ethernet or Wi-Fi) for delivery. Just like a layer-2 network uses layer-1 links to actually move the bits.

The difference in moving data at layer 1, 2 or 3 is the complexity of the devices. Layer-1 devices (repeaters) just copied bits - simple, yet inefficient and long obsolete. Layer-2 devices (switches) intelligently forward frames and are very efficient, but layer-2 networks are limited in size. Layer-3 devices (routers) can handle networks of (theoretically) any size but their complexity initially prohibited building them in hardware. Software routers were slow and expensive. Of course, a router needs to have hardware for layers 2 and 1 as well.

Even though it seems that Layer 2 access layer solutions are slowly being replaced by blazing fast Layer 3 solutions.

This is about something different. The author describes the basic network architecture over time. Formerly, only layer-2 networks were used within a company network connected to the outside by (slow and expensive) routers. As technology advanced, routers became hardware-based and much faster. On the other hand, switches became more advanced and learned basic routing (layer-3 switching).

At the same time, networks grew much larger than they used to be, rendering the "layer 2 throughout" design obsolete - layer-2 networks only scale in a limited way.

With performance up and cost down, it became common design to use routers and layer-3 switch in the core. Today, the layer-3 approach has grown outside from the core into the aggregation layer, enabling even larger networks. The next step is to also use routing instead of switching in the access layer where the clients are connected.

Spanning Tree Protocol (STP) is very useful to organize your layer-2 connections. It takes care of preventing bridge loops in case of redundant mesh links in your network - bridge loops usually cause broadcast storms and MAC table instabilities that can take down your entire network.

With a network using layer-3 connections exclusively, you wouldn't need STP - but that would mean that each link is a subnet of its own and all connections between links are routed. While this is reasonable between the core and the aggregation layer and possibly between aggregation and access, it's rather pointless on the access layer towards the clients.

It used to be, "Switch where you can, route where you must." That has been turned on its head. Of course, an interface, even on a layer-3 router, uses both layer-1 and layer-2 protocols. I think the idea is that the current best practices are moving toward routing all the way to the access switch. Within the access switch, you are still layer-2, but the access switches connect to the distribution via layer-3 connections and routing, and access switches do not connect to each other.

That means that a VLAN exists only on the single access switch, You can have multiple VLANs on a single access switch, but those VLANs do not extend to any other access switches. This will almost completely remove any STP or other layer-2 problems, e.g. broadcast storms, that can bring an entire site to its knees. We live in a layer-3 world, and it is now rare to have an application that needs the same VLAN across all the hosts. We route across the Internet, so why not route in your own network?

The reason it has been changing is that we now have a lot of layer-3 switches used as access switches that can route to the distribution.

The quote you quoted is simply misleading. All modern network devices transmit data at wire speed, so they're all "blazing fast." Whether it's layer 2 or 3 makes no difference in data rates.

A host will create data, and then encapsulate it in a lower level protocol data unit. So TCP gets encapsulated in IP, which gets encapsulated in 802.11, etc. It's all 1's and 0's when it leaves the computer. The receiving host reverses the process and de-encapsulates the data as it moves up the protocol software stack.