In general, yes, a user on any mastodon instance can access any other - they are not siloed.
Even further: a user on a mastodon instance can see & interact with posts on Lemmy (the Reddit alternative), pics on Pixelfed (=Instagram), videos on Peertube (=Youtube), and content from a long list of other services, all from their single mastodon instance.
(There are exceptions, e.g. you could run a siloed internal company mastodon, and notably server administrators are able to block other servers entirely - but as a user you can always choose a server that federates as widely as you'd like, and if you self-host you're unlikely to block or be blocked by anybody)
Even further: a user on a mastodon instance can see & interact with posts on Lemmy (the Reddit alternative), pics on Pixelfed (=Instagram), videos on Peertube (=Youtube), and content from a long list of other services, all from their single mastodon instance.
(There are exceptions, e.g. you could run a siloed internal company mastodon, and notably server administrators are able to block other servers entirely - but as a user you can always choose a server that federates as widely as you'd like, and if you self-host you're unlikely to block or be blocked by anybody)