We can share SharePoint Online sites with external users like vendors or clients in different domains. This helps to collaborate with vendors and clients on the same SharePoint site. Sometimes solutions need some customizations in list and library forms. In this case, you can create custom form ins SPFx or Canvas app in PowerApps. The issue with Canvas apps in Power Apps is that external users should have a Power Apps license, either provided by your company or their own company. Without a license, they can not access Power Apps Canvas apps. There is a solution to this problem. As per Microsoft, if you have only SharePoint as a data source and do not use any premium connection in your app, you can customize SharePoint list forms directly from SharePoint using the Customize with Power Apps option.
Share SharePoint online site with external users
First, you must enable external sharing of your SharePoint online site with external users. To do this, you have to do the following:
Open the SharePoint Central Admin site and select the site where you want to enable external sharing. Go to the Policies section.
Edit External sharing. Select 'New and existing guests' in Site content can be shared with. Check Limit sharing by domain. Add the domain of your external users. This will ensure that only users from that particular external domain can access the site. You can add more than one domain in case of multiple vendors and clients accessing the same SharePoint site.
When you add domains to your SharePoint online site, you can also block specific domains.
Once you have enabled external sharing of your SharePoint online site, you can customize the SharePoint list/ library form using Customize with Power Apps.
As per Microsoft, you need the following licensing to share Power Apps forms/ apps with external or guest users. You don't need any license for Customized Microsoft Lists or SharePoint library form (using non-premium connectors). So if you have just SharePoint as a data source without any other premium connector, you can share your custom Power Apps list/ library form with your external or guest users, as you can see in the highlighted portions in the below image.