We can use Schema.getGlobalDescribe() to get all properties of sObject and its fields.

Apex Class:

public class SampleController {
    
    Public string selectedObj {get;set;}
    public String selectedField {get; set;}
    
    Public List<Selectoption> getObjList(){
        List<Schema.SObjectType> objList = Schema.getGlobalDescribe().Values();     
        List<SelectOption> objNames = new List<SelectOption>();
        objNames.add(new SelectOption('','-- Select --'));
        for(Schema.SObjectType obj : objList)
        {
            objNames.add(new SelectOption(obj.getDescribe().getName(),obj.getDescribe().getLabel()));
        }
        objNames.sort();
        return objNames;
    }
    
    public List<SelectOption> getObjectFields() {
        List<SelectOption> fieldNames = new List<SelectOption>();
        fieldNames.add(new SelectOption('','-- Select --'));
        if(!String.isBlank(selectedObj)){
            Map<String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
            Schema.SObjectType ObjSchema = schemaMap.get(selectedObj);
            Map<String, Schema.SObjectField> fieldMap = ObjSchema.getDescribe().fields.getMap();
            for (String fieldName: fieldMap.keySet()) 
            {  
                fieldNames.add(new SelectOption(fieldName, fieldMap.get(fieldName).getDescribe().getLabel()));
            }
        }
        return fieldNames;
    }      
}

Visualforce Page:

<apex:page controller="SampleController">
    <apex:form> 
        <apex:pageBlock>
            <apex:pageBlockSection columns="2">
                
                <apex:pageBlockSectionItem >
                    <apex:outputlabel value="Object Names :"/> 
                    <apex:actionRegion >      
                        <apex:selectList value="{!selectedObj}" size="1">
                            <apex:selectOptions value="{!ObjList}"/>
                            <apex:actionSupport event="onchange" rerender="objFields"/>
                        </apex:selectList>
                    </apex:actionRegion>                         
                </apex:pageBlockSectionItem>
                
                <apex:pageBlockSectionItem >
                    <apex:outputlabel value="Field Names :"/>   
                    <apex:outputPanel id="objFields">   
                        <apex:actionRegion >  
                            <apex:selectList value="{!selectedField}" size="1">
                                <apex:selectOptions value="{!ObjectFields}"/>
                            </apex:selectList>
                        </apex:actionRegion>      
                    </apex:outputPanel>
                </apex:pageBlockSectionItem>
                
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>

Output: