Cannot find the UseMysql method on DbContextOptions

Use the below code if you are referencing the package Pomelo.EntityFrameworkCore.MySql

 services.AddDbContextPool<AppDBContext>(options =>
        {
            var connetionString = Configuration.GetConnectionString("DefaultConnection");
            options.UseMySql(connetionString, ServerVersion.AutoDetect(connetionString));
        });

You need

using Microsoft.EntityFrameworkCore;
using MySQL.Data.EntityFrameworkCore.Extensions;

Oracle is not complying to the standard practices when using Dependency Injection, so its all a bit different. The standard practice is to put the extension methods for Depedency Injection into Microsoft.Extensions.DependencyInjection namespace, which is included in most ASP.NET Core app projects so the method becomes automatically available when a package is imported.