Always On Failover Cluster Instances (FCI) on existing SQL Server

I have launched three EC2 Windows_Server-2019_SQL_2019_Standard instances. How can I implement Always On Failover Cluster Instances (FCI) on these existing SQL Servers. I can only find details like when you install SQL first time that time only you can choose the Failover Clustering option.

Don't get confused FCI with AlwaysON FCI. AlwaysON is just a marketing term used by MS.
I think your goal is to implement Availability groups?
If that's the case, you just perform a normal standalone SQL installation on all the cluster nodes and proceed with AG configuration.
You can also have FCI+AG setup, but for the FCI the SQL installation is different.

As you are on standard version, you can only use basic availability groups.

Exactly, since I have Standard SQL Server I want to implement FCI not AG.
I will be using FSx as well.
Is it possible to do FCI once SQL Server installation is done?

You can - but then you have to install SQL Server again into the FCI. For an FCI you create the cluster - with shared storage and a witness (if needed). Then - you install SQL Server into the FCI using the cluster installation.

For an AG - you install standalone instances of SQL Server on each node outside the cluster.

Thank you, this helps.